http://www.gameranx.com/updates/id/16057/article/rockstar-reveals-more-information-on-gta-v-s-animals/

-Customize your dog (collar, other accessories, etc)
-Your dog's name is Chop
-Keep an eye on your dog, you can lose him
-Over 15 types of wild animals can be found throughout the world
-Each character has a specific walking animation for individual realism
-Overall animations have a been a focus of improvement for Rockstar
-There is a dedicated underwater mission that Michael and Trevor go to via jet-skis together