To clarify, The Sims 3 Pets isn’t playable at E3. Instead, they have the Create-A-Pet running as a separate tool. Anton over at GameMarshal wrote their impressions on creating a pet:
The actual customizations themselves are very thorough. Everything from choosing your breed down to the two tone eye colors, stripe colors, fur shading and more. You can “re-make” your real-life pet companion, or go crazy and create something out of this world. Whatever color and style you decide to go with for your pet, you’ll also be able to assign various characteristics and traits to it. All of these characteristics come with unique animations providing you a more real-life experience. As an example, give an aggressive personality to your pet horse and you’ll be able to kick a Sim in the chest if they get too close for comfort. Or, make your cat a destructive one and watch chaos unfold once you decide to scratch up that favorite couch. All of these characteristics you choose will have a direct impact of how you play the game and interact with your Sims.