An animal is a living thing. Even with that, things get tricky to pin down just what an animal is.
Biologists will tell you animals consume organic material, breathe oxygen, are able to move, can reproduce sexually. That is, except in a few cases. That's downright cheating if you ask me.
Intelligent design advocates will tell you animals are things that walk and crawl upon the earth. But that doesn't explain the sponge that comes alive during your shower and tries to eat you.
Animals are not to be confused with creatures of myth or legend even though the Intelligent Design folks consider unicorns and leprechauns to be real. It is possible to confuse, say, the Easter Bunny, with real animals when that character has been in existence for so many years. Okay, okay, just stop crying. The Easter Bunny IS real. Jeez.