Okay so i did make a mistake of buying my two guinea pig boys from a local pet store but they did come from licensed breeders. When i brought them home i noticed behind CeeLo's ear he was missing hair like a patch behind both and it was very dark and like dry dark skin. My other guinea pig Charlie is getting it behind one of his ears so i don't think its like from them fighting with other guinea pigs before i got them. They are both about 5 months old. I have noticed they do like itch behind there ears or somewhere else so im also thinking it would be like mites or lice or something.