Question: When Virgil wrote "quos ego", what branch of the Indo-European languages was he using?

Context paragraphs:
Indo-Aryan languages: The Indo-Aryan or Indic languages are the dominant language family of the Indian subcontinent.  They constitute a branch of the Indo-Iranian languages, itself a branch of the Indo-European language family.  Indo-Aryan speakers form about one-half of all Indo-European speakers (about 1.5 of 3 billion), and more than half of all Indo-European languages recognized by "Ethnologue".  While the languages are primarily spoken in South Asia, pockets of Indo-Aryan languages are found to be spoken in Europe and the Middle East.

Germanic substrate hypothesis: The Germanic substrate hypothesis is an attempt to explain the distinctive nature of the Germanic languages within the context of the Indo-European languages.  It claims that the elements of the Common Germanic vocabulary and syntactical forms, which do not seem to have cognates in other Indo-European languages, suggest that Proto-Germanic may have been either a creole or a contact language that subsumed a non-Indo-European substrate language or a hybrid of two quite different Indo-European languages, from the centum and satem types, respectively.

Warren Cowgill: Warren Cowgill ( ; 1929–1985) was an American linguist.  He was a professor of linguistics at Yale University and the Encyclopædia Britannica's authority on Indo-European linguistics.  He was unusual among Indo-European linguists of his time in believing that Indo-European should be classified as a branch of Indo-Hittite, with Hittite as a sister language of the Indo-European languages, rather than a daughter language.

Ancient Anatolians: The Anatolians were a group of distinct Indo-European peoples who spoke the Anatolian languages and shared a common culture.  The Anatolian languages were a branch of the larger family of Indo-European languages.  According to the most widely accepted Kurgan theory on the Proto-Indo-European homeland, however, these Indo-European Anatolians were themselves immigrants to Anatolia from the north.

List of extinct languages of Europe: This is a list of extinct languages of Europe, languages which have undergone language death, have no native speakers and no spoken descendant.  As the vast majority of Europeans speak Indo-European languages, a result of the westward portion of the prehistoric Indo-European migrations, the bulk of the indigenous languages of Europe became extinct thousands of years ago without leaving any record of their existence as they were replaced with Celtic, Italic, Germanic, Balto-Slavic, Hellenic, and Iranian Indo-European languages.  A small minority of these extinct languages, however, survived long enough to be attested.

Indigenous Aryans: The Indigenous Aryans theory, also known as the Out of India theory, proposes that the Indo-European languages, or at least the Indo-Aryan languages, originated within the Indian subcontinent, as an alternative to the established migration model which proposes the Pontic steppe as the area of origin of the Indo-European languages.  The indigenist view sees the Indo-Aryan languages as having a deep history in the Indian subcontinent, and being the carriers of the Indus Valley Civilization.  This view proposes an older date than is generally accepted for the Vedic period, which is generally considered to follow the decline of Harappan culture.

Quos ego: Quos ego (Latin, literally 'Whom I') are the words, in Virgil's "Aeneid" (I, 135), uttered by Neptune in threat to the disobedient and rebellious winds.  Virgil's phrase is an example of the figure of speech called aposiopesis.

Pre-Indo-European languages: Pre-Indo-European languages are any of several old languages, not necessarily related to one another, that existed in prehistoric Europe and South Asia before the arrival of speakers of Indo-European languages.  The oldest Indo-European language texts date from 19th century BCE in Kültepe in modern-day Turkey, and while estimates vary widely, spoken Indo-European languages are believed to have developed at the latest by the third millennium BCE (see Proto-Indo-European Urheimat hypotheses).  Thus the Pre-Indo-European languages must have developed earlier than, or in some cases alongside, the Indo-European languages.

Latin: Latin (Latin: "lingua latīna" , ] ) is a classical language belonging to the Italic branch of the Indo-European languages.  The Latin alphabet is derived from the Etruscan and Greek alphabets, and ultimately from the Phoenician alphabet.

Illyrian languages: The Illyrian languages are a group of Indo-European languages that were spoken in the western part of the Balkans in former times by groups identified as Illyrians: Ardiaei, Delmatae, Pannonii, Autariates, Taulanti (see list of ancient tribes in Illyria).  Some sound changes from Proto-Indo-European to Illyrian and other language features are deduced from what remains of the Illyrian languages, but because there are no examples of ancient Illyrian literature surviving (aside from the Messapian writings if they can be considered Illyrian), it is difficult to clarify its place within the Indo-European language family.  Because of the uncertainty, most sources provisionally place the Illyrian language family on its own branch of Indo-European, though its relation to other languages, ancient and modern, continues to be studied.

Supporting paragraphs: Quos ego Latin
Answer: Italic