As far as I can tell from their appearance, they're pretty much like woolly mammoths: Long shaggy fur, slow moving and high fat content. This type of biology would be better suited to planets like Ilum or Hoth, but yet Banthas are an integral part of Tatooine culture. How is it that they survive on this desert planet when their bodies' design wouldn't work with the high temperatures and sandstorms? (In contrast, other animals on Tatooine seem much more suitable to the environment: Eopies, Krayt dragons and Dewbacks.)
A group of science bloggers recently participated in a "Science of Tatooine" blog carnival, writing pieces of scholarly speculative fiction. It's not canon, but canon doesn't have much to say about them.
John Hutchinson wrote "The Functional Anatomy of Tatooine Megafauna" and has this to say about banthas:
There's more to it, and he does build on Terryl Whitlach's (assumedly canon) artwork.