Why did Egyptian Gods have animal heads?
Egyptian gods were often depicted in therianthrophic – part human, part animal form, to depict the personality of that particular god/ess in a symbolic way. For example, Sekhmet, goddess of ferocious war, was sometimes shown with the head a lioness, as lions are ferocious creatures. Similarly Anubis was shown with a jackal head because the jackal was associated with the necropolis and Anubis was a god of the dead.
There are also theriomorphic depictions, where gods are shown entirely in animal form. These are quite common, and in fact were the most common representations of gods in the very earliest periods of Egyptian history. For example, Anubis as a black jackal, or Thoth as either an ibis or a white baboon. Taweret was even a hybrid of hippo, crocodile and lioness.
It is the same reasoning behind why Christians equate Jesus with the lamb, or why we give angels wings.