Beast Forms
The animal forms like Cow and Spider are intended as curses, but they each have their own play styles, so some players enjoy playing as animals full time.
Animal Woes
Observant players have noticed that it's hard to craft gear for animal-cursed players. There's only one gem that covers all animal curses, so you often craft Deer gear when you really want Spider gear, and so on.
In the next update (edit me), there will be a way to inscribe runes into those gems, so that you can attune the gem to one specific animal curse. You'll want to get your Sigil Scripting skill to level 25 to be able to use those techniques.
Returning to Human Form (temporarily)
In order to make "being an animal" a viable path through the entire game, the next update (Sept 2014) adds some high-level Words of Power that can help. These Words apply an illusion to you that makes you look like a human for a minute or so -- just long enough to talk to NPCs that ignore animals!
Forms
The following Beast Forms are available: