I'm pretty much with Rapaire on this one, although choice of animal can depend on the size of lawn and general area.
Rabbits in a frame with netting on top and no underside are pretty efficient on a smaller area. Sheep or goats can clear a larger space, although goats will eat anything they can reach - they are particularly fond of privet, which is poisonous to them. Ducks or geese are also pretty efficient.
Quack!
GtD.