Have you ever seen a centaur at the zoo? I doubt it! It's a mythological creature that's half-man, half- horse.
Like elves, trolls, werewolves, zombies, and vampires, the centaur is a mythical creature, specifically a creature equal parts horse and man. This is a little like a mermaid, who is half-fish and half-woman. In case you were wondering, the top half is the man part. Even in the made-up world of mythology, the other way around just wouldn't work.