The same rules apply for animals. Including #2 in some parts of the world.
1) To expel waste matter (feces, dogs eggs) from the body.
2) To provide an alternative orifice on a lady/animal for penis insertion. Or as the main orifice for men.
3) To be used as an insult
The anus is also known as the asshole, butthole, chocolate starfish, brown eyed willy and has many other slang terms for it.
Last night i was having sexual relations with my girlfriend, and i proceeded to insert my finger into her anus.