Why would flies land on an animal? and about Guinea pig..?

Flies are attracted to open wounds because they lay their eggs in there. The fly larvae (maggots) eat decayed or rotting flesh. If the guinea pig has an open wound, that would attract flies. If the guinea pig had waste on it, that would attract flies. I'd carefully look over the animal and make sure he didn't have any cuts or scs and then take him and clean him up.