Aren't there some dwarf pig breeds that don't get bigger than around a cat or rabbit?
Try medium to large size dog, they're just breeds not bred for commercial meat, some were specialised for agricultural purposes (turning over and fertilising soil). Dwarfism does occur, but isn't something you can guarantee is passed down, and the true dwarf micro pigs I've heard of all had heart conditions and the like that limited their life spans. Other cases were malnutrition during development rather than true dwarfism.
Instead of hoping for a pig that stays a piglet forever, we should consider how big wild species are and that even one larger than a cat can be smaller and more manageable. Pigs are good companions, they're just not small pets.