The Sims 3: Pets expansion pack adds the ability to own cats and dogs. You can customize their breeds, personalities, and even train them to perform tricks. You can also experience the joys and challenges of pet ownership, including feeding, grooming, and taking your pets for walks.
There's also a separate expansion pack called The Sims 3: Supernatural which adds a range of supernatural creatures you can have as pets, including fairies, vampires, and even zombies!
So, if you're looking to add furry (or even spooky) companions to your Sims' lives, be sure to grab those expansion packs.