I hope this doesn't sound too silly, but the human traits, Loves/Hates the Outdoors, should also be a pet trait exclusively for cats. It could really define whether a cat is an indoor or an outdoor type, just like the real world. I could see them getting that mood benefit from being in or outside, like their owners do. Perhaps they can supposedly get along with Sims that have the same trait, same way supposedly a clueless pet gets along with absent minded and easily impressed Sims, as well as nervous horses supposedly get along with Neurotic Sims.