It can be any animal you’d like, real, fictional, mythical, magical, whatever. Just truly embody that animal for five minutes. Think like that animal, move like that animal, experience life like that animal. For example: a mouse’s view of the world would be considerably different than a human’s, given the 5-6 foot height difference. Release all your inhibitions—after all, it’s not like anyone’s watching—an just become that animal.
This exercise will teach you to get over any embarrassments you may have about acting wild. It will also help you to learn how to truly become a character and start up an inner monologue (though, it is a given that an animal’s inner monologue is probably a bit more…simplistic than a human’s would be).