This is quite common in young dogs and it's a red, hairless benign skin tumor usually on the head, one eye, ears or limbs and tumor will spontaneously recede without treatment in 2 - 3 months. Histiocytoma is painless. It sounds as if you have already been to the vets and been given a diagnosis (your vet is keeping an eye on it) and if you haven't been to a vet with your dog please go and get a proper diagnosis.