PET is a three-letter word that can be used to describe either a kitty or doggie. A kitty, also known as a cat, is a small, domesticated mammal that is often kept as a PET. A doggie, also known as a dog, is a domesticated mammal that is often kept as a PET and is known for its loyalty and companionship. Both kitties and doggies are popular pets and can provide their owners with love and affection.