I once found a snake fast asleep under the plastic sheeting covering the compost heap. More recently rats have tunneled into the compost possibly feasting on the spent grain from the chickens.
They have even eaten through a plastic dustbin where I kept the chicken feed. The trouble is, if you don't deal with them swiftly, they multiply like crazy.
I once threw a stone at one to scare it away and being a better shot than I thought killed it outright - a bit like Mel Gibson in Braveheart.