I am afraid it is not possible to properly identify this growth from a photo. This could be a wart, a cutaneous horn (both benign), or a tumor or just a tissue reaction. In order to identify such a growth we need to palpate it, determine whether it is solely in the skin or attached under the skin, movable against certain tissues, has a discharge, compare the visible size with the size in or under the skin, take a fine needle biopsy, etc. and give the dog a general exam. If this growth appeared suddenly or appears to be growing, I suggest you take Oskar to your vet to have it examined. Old dogs often get tumors, most of the time they are benign warts, but sometimes they can be bad cancer.