<PAGE>
EXHIBIT 4.3 FORM OF WARRANT

THIS WARRANT AND ANY SHARES ISSUABLE UPON THE EXERCISE OF THIS WARRANT HAVE NOT
BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 (THE "ACT"), NOR UNDER ANY
STATE SECURITIES LAW AND SUCH SECURITIES MAY NOT BE PLEDGED, SOLD, ASSIGNED,
HYPOTHECATED, OR OTHERWISE TRANSFERRED UNTIL (1) A REGISTRATION STATEMENT WITH
RESPECT THERETO IS EFFECTIVE UNDER THE ACT AND ANY APPLICABLE STATE SECURITIES
LAW OR (2) THE COMPANY RECEIVES AN OPINION OF COUNSEL TO THE COMPANY OR COUNSEL
TO THE HOLDER OF SUCH SECURITIES, WHICH COUNSEL AND OPINION ARE REASONABLY
SATISFACTORY TO THE COMPANY, THAT SUCH SECURITIES MAY BE PLEDGED, SOLD,
ASSIGNED, HYPOTHECATED, OR TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION
STATEMENT UNDER THE ACT AND APPLICABLE STATE SECURITIES LAWS.

THE TRANSFER OF THIS WARRANT AND THE SHARES ISSUABLE UPON THE EXERCISE HEREOF
ARE RESTRICTED AS DESCRIBED HEREIN.

THE SALE, ASSIGNMENT, TRANSFER, PLEDGE AND OTHER DISPOSITION OF THIS WARRANT AND
THE SHARES ISSUABLE UPON THE EXERCISE OF THIS WARRANT ARE RESTRICTED BY THE
INVESTORS' RIGHTS AGREEMENT (THE "INVESTORS' RIGHTS AGREEMENT"), DATED AS OF
DECEMBER 1, 1999. A COPY OF THE INVESTORS' RIGHTS AGREEMENT IS ON FILE WITH THE
CORPORATE SECRETARY AT THE PRINCIPAL EXECUTIVE OFFICES OF THE COMPANY. A COPY
THEREOF MAY BE OBTAINED AT NO COST UPON WRITTEN REQUEST THEREFOR MADE BY THE
HOLDER OF RECORD OF THIS CERTIFICATE TO THE CORPORATE SECRETARY AT THE PRINCIPAL
OFFICES OF THE COMPANY.

                                  ON2.COM INC.
            WARRANT TO PURCHASE             SHARES OF COMMON STOCK,
                                  NO PAR VALUE

No.W-                                                          December 30, 1999

    THIS CERTIFIES that, for good and valuable consideration, the receipt and
    sufficiency of which are hereby acknowledged,             (the "Holder"), is
    entitled to subscribe for and purchase from On2.com Inc., a Colorado
    corporation (the "Company"), upon the terms and conditions set forth herein,
    at any time or from time to time, during the period commencing on the date
    set forth above and expiring at 5:00 p.m. on December  , 2004 (the "Exercise
    Period"),             (            ) shares of the Company's Common Stock,
    no par value per share (the "Common Stock"), at an exercise price (the
    "Exercise Price") per share equal to $      . As used herein, the term "this
    Warrant" shall mean and include this Warrant and any Warrant or Warrants
    hereafter issued as a consequence of the exercise or transfer of this
    Warrant in whole or in part. As used herein, the term "Holder" shall include
    any transferee to whom this Warrant has been transferred in accordance with
    the terms hereof.

    The number of shares of Common Stock issuable upon exercise of this Warrant
(the "Warrant Shares") and the Exercise Price may be adjusted from time to time
as hereinafter set forth in Section 6.

    1. This Warrant may be exercised during the Exercise Period, as to the whole
or any lesser number of whole Warrant Shares, by transmission by telecopy of the
Election to Exercise, followed within three (3) business days by the surrender
of this Warrant (with the Election to Exercise attached hereto duly executed) to
the Company at its office at 375 Greenwich Street, New York, New York 10013, or
at such other place as is designated in writing by the Company, together with a
certified or bank cashier's check payable to the order of the Company in an
amount equal to the product of the Exercise Price and the number of Warrant
Shares for which this Warrant is being exercised (the "Aggregate Exercise
Price").

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    2. Upon each exercise of the Holder's rights to purchase Warrant Shares, the
Holder shall be deemed to be the holder of record of the Warrant Shares issuable
upon such exercise, notwithstanding that the transfer books of the Company shall
then be closed or certificates representing such Warrant Shares shall not then
have been actually delivered to the Holder. Within five (5) business days after
each such exercise of this Warrant and receipt by the Company of this Warrant,
the Election to Exercise and the Aggregate Exercise Price, the Company shall
issue and deliver to the Holder a certificate or certificates for the Warrant
Shares issuable upon such exercise, registered in the name of the Holder or its
designee. If this Warrant should be exercised in part only, the Company shall,
upon surrender of this Warrant for cancellation, execute and deliver a new
Warrant evidencing the right of the Holder to purchase the balance of the
Warrant Shares (or portions thereof) subject to purchase hereunder.

    3. Any Warrants issued upon the transfer or exercise in part of this Warrant
shall be numbered and shall be registered in a Warrant register (the "Warrant
Register") as they are issued. The Company shall be entitled to treat the
registered holder of any Warrant on the Warrant Register as the owner in fact
thereof for all purposes and shall not be bound to recognize any equitable or
other claim to or interest in such Warrant on the part of any other person, and
shall not be liable for any registration of transfer of Warrants which are
registered or to be registered in the name of a fiduciary or the nominee of a
fiduciary unless made with the actual knowledge of the general counsel of the
Company that a fiduciary or nominee is committing a breach of trust in
requesting such registration of transfer. In all cases of transfer by an
attorney, executor, administrator, guardian, or other legal representative, duly
authenticated evidence of his or its authority shall be produced. Upon any
registration of transfer, the Company shall deliver a new Warrant or Warrants to
the person entitled thereto. This Warrant may be exchanged, at the request of
the Holder thereof, for another Warrant, or other Warrants of different
denominations, of like tenor and representing in the aggregate the right to
purchase a like number of Warrant Shares (or portions thereof), upon surrender
to the Company or its duly authorized agent. Notwithstanding anything contained
herein to the contrary, the Company shall have no obligation to cause Warrants
to be transferred on its books to any person if, in the opinion of counsel to
the Company, such transfer does not comply with the provisions of the Act and
the rules and regulations thereunder.

    4. The Company, until the expiration or termination of this Warrant, shall
reserve and keep available out of its authorized and unissued common stock,
solely for the purpose of providing for the exercise of the rights to purchase
all Warrant Shares granted pursuant to this Warrant and all other Common Stock
Warrants, such number of shares of common stock as shall, from time to time, be
sufficient therefor. The Company covenants that all shares of stock issuable
upon exercise of this Warrant, upon receipt by the Company of the full Exercise
Price therefor, shall be validly issued, fully paid, nonassessable, and free of
preemptive rights.

    5. The issuance of any Warrant, Warrant Shares or other securities upon the
exercise of this Warrant, and the delivery of certificates or other instruments
representing such Warrant Shares or other securities, except as otherwise
required by law, shall be made without charge to the Holder for any tax or other
charge in respect of such issuance, other than applicable transfer taxes.
Notwithstanding anything contained herein, all applicable transfer taxes shall
be borne by the Holder.

    6. The number of Warrant Shares and the Exercise Price shall be subject to
adjustment from time to time as provided in this Section.

    6.1 If, during the Exercise Period, the Company shall pay or make a dividend
       or other distribution on any class of capital stock of the Company in
       Common Stock, the number of Warrant Shares shall be increased by
       multiplying such number of shares by a fraction of which the denominator
       shall be the number of shares of Common Stock outstanding at the close of
       business on the day immediately preceding the date of such distribution,
       and the numerator shall be the sum of (a) such number of shares and
       (b) the total number of shares constituting such dividend or other

                                       2
<PAGE>
       distribution, such increase to become effective immediately after the
       opening of business on the date following such distribution.

    6.2 If, during the Exercise Period, the outstanding shares of Common Stock
       shall be subdivided into a greater number of shares of Common Stock, the
       number of Warrant Shares at the opening of business on the day following
       the day upon which such subdivision or combination becomes effective
       shall be proportionately increased, and, conversely, if outstanding
       shares of Common Stock shall each be combined into a smaller number of
       shares of Common Stock, the number of Warrant Shares at the opening of
       business on the day following the day upon which such combination becomes
       effective shall be proportionately decreased, such increase or decrease,
       as the case may be, to become effective immediately after the opening of
       business on the day following the day upon which such subdivision or
       combination becomes effective.

    6.3 The reclassification of Common Stock into securities (other than Common
       Stock) and/or cash and/or other consideration shall be deemed to involve
       a subdivision or combination, as the case may be, of the number of shares
       of Common Stock outstanding immediately prior to such reclassification
       into the number or amount of securities and/or cash and/or other
       consideration outstanding immediately thereafter, and the effective date
       of such reclassification shall be deemed to be "the day upon which such
       subdivision becomes effective" or "the day upon which such combination
       becomes effective," as the case may be, within the meaning of
       Section 6.1.

    7. Upon receipt of evidence satisfactory to the Company of the loss, theft,
destruction, or mutilation of any Warrant (and upon surrender of any Warrant if
mutilated), and upon reimbursement of the Company's reasonable incidental
expenses and, if reasonably requested, an indemnity reasonably acceptable to the
Company, the Company shall execute and deliver to the Holder thereof a new
Warrant of like date, tenor, and denomination.

    8. The Holder of any Warrant shall not have, solely on account of such
status, any rights of a stockholder of the Company, either at law or in equity,
or to any notice of meetings of stockholders or of any other proceedings of the
Company, except as provided in this Warrant.

    9. This Warrant shall be governed by and construed in accordance with the
laws of the State of Colorado, without giving effect to the rules governing the
conflicts of laws.

    10. The parties hereby irrevocably consent to the jurisdiction of the courts
of the State of New York and of any federal court located in such State in
connection with any action or proceeding arising out of or relating to this
Warrant, any document or instrument delivered pursuant to, in connection with,
or simultaneously with this Warrant, or a breach of this Warrant.

    IN WITNESS WHEREOF, the undersigned has caused this Warrant to be duly
executed by its officers thereunto duly authorized as of the date and year set
forth below.

Dated: December 30, 1999

<TABLE>
<S>                                                    <C>  <C>
                                                       ON2.COM INC.

                                                       By:  --------------------------------------
                                                            Name: Daniel B. Miller
                                                            Title: President and Chief Executive
                                                            Officer
</TABLE>

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<PAGE>
                               FORM OF ASSIGNMENT

(To be executed by the registered holder if such holder desires to transfer the
attached Warrant.)

    FOR VALUE RECEIVED,                         hereby sells, assigns, and
transfers unto             a Warrant to purchase             shares of Common
Stock, no par value per share, of On2.com Inc. (the "Company"), together with
all right, title, and interest therein, and does hereby irrevocably constitute
and appoint             attorney to transfer such Warrant on the books of the
Company, with full power of substitution.

Dated: ____________

                                          Signature ____________________________

Signature Guaranteed:

                                     NOTICE

    The signature on the foregoing Assignment must correspond to the name as
written upon the face of this Warrant in every particular, without alteration or
enlargement or any change whatsoever.

                                       4
<PAGE>
To:  On2.com Inc.

                              ELECTION TO EXERCISE

    The undersigned hereby exercises his or its rights to purchase       Warrant
Shares covered by the within Warrant and tenders payment herewith [in the amount
of $            ] in accordance with the terms thereof, certifies that he owns
this Warrant free and clear of any and all claims, liens and/or encumbrances and
requests that certificates for such securities be issued in the name of, and
delivered to:

                    (Print Name, Address and Social Security
                         or Tax Identification Number)

and, if such number of Warrant Shares shall not be all the Warrant Shares
covered by the within Warrant, that a new Warrant for the balance of the Warrant
Shares covered by the within Warrant be registered in the name of, and delivered
to, the undersigned at the address stated below. Dated: _____    Name: _________
                                                       (Print)

Address: _______________________________________________________________________

(Signature)

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