Thanks to callum91 of our forums, we have some new information. As has been the case with the pre-order items for various expansions you are given a code and a url to go to which allows you to download said bonus item. However, with the Pets expansion they’re changing things up a bit!

As quoted from the order info on the official site:

Purchase The Sims 2 Pets from one of the retailers above to unlock an exclusive pet for your Sims! After you purchase the game you’ll receive a code plus directions letting you know how to use the code to unlock the pet within the game itself (this is not a downloadable item). This special offer is only available for a limited time while supplies last, so buy your copy of The Sims 2 Pets and secure your exclusive pet today!

Talk about it on our forums.