Conversation so far:
What was pre-crisis DC Comics' Comet?
DC Comics' Comet was Supergirl's pet horse and, while in his human form as Bill Starr, her brief boyfriend.
What was DC Comics' Comet like pre-crisis?
Comet was originally a Grecian centaur named Biron, turned horse by the witch Circe who originally wanted to turn him into a human. She gave him superpowers, including immortality.

Question: how did he change?
Self-contained rewrite: How did DC Comics' Comet change into human form?