Horns or Antlers are sometimes used for God images, as are statues of ancient Roman and Greek mythologies. A seashell can be used as a Goddess symbol, as can several other natural objects. The sky is the limit on this, for we must each decide for ourselves how we see our Gods. Just about anything that you can find may be used as a representation of your Deity. An altar is merely a place or object upon which sacred items are placed for the purpose of worship or ritual.
Altars can be anything, from a coffee table to a tree stump.
Some Pagans and Wiccans claim that the altar must be round to represent the circle and others claim that there is no particular shape that an altar must be. Again this is entirely up to you. No one has the right to tell you that your altar is the wrong kind or shape. Anything that you find where you can place your ritual items and it works for you, can be considered an altar. Altars have always been used as a focal point for worship or ritual.