guinea pig horn

My guinea pig has an odd bulge and his nipple is abnormally large. He also has a wound on the bulge. Do you know what this could be or how to treat it?

[becks715]

It could many things most common being infection which stemmed from it being cut or bit.  Get him to a vet ASAP.  Guinea pigs are very fragile and any injury could be worse then it seems they hide their sickness and injury as a natural instict from being pray in the wild.  Vet vet vet.