Found some weird pokemon wiki and this is what snorlax eats. So he probably eat cats to.
Snorlax will eat anything and everything that is edible, they aren't really very picky. Upon finding something new on its territory, a resident Snorlax will put it in its mouth to see if it's something that can be eaten. Snorlax are fond of large, thorny vines as a snack item.