Nothing new, really. The folks at Gamespot were able to play around with the Pets creator at the EA 2011 Summer Showcase. Here is their report:
One of the biggest changes to this expansion is that you have full control over your pets. You can choose to play as your pet if you choose, mingle with other animals and socialize with your owner. We weren’t able to see how these animals work in-game since the build we were playing with was still in pre-alpha, but we did get an idea of how in-depth you could go with the creator. You will have access to more than a hundred breeds of cats and dogs, and then you can further customize their fur and the shape of virtually any body part. You can re-create your childhood pal, or conjure up some kind of four-legged freak–the option is there!