Any variety of buttercup (or in fact pretty much any member of the ranunculaceae family, including buttercups and delphinium) is poisonous to many mammals.
The one to specifically look out for is creeping buttercup, which is quite common on lawns. They spread by making offshoots, so you often have got quite a cluster.
You'd best to a thorough check of your lawn and dig out any you come across; you can eliminate them over the course of a few years.
You need to worry if your piggy is eating a just little; that won't kill. But it is not something you want your piggies to expose to if at all possible.