It's not easy to judge a two dimensional photo, but it appears likely to me to be a giant feather cyst. It looks like it's just under the top layer of tissue. Does it appear translucent around the base to you?
Try moving the mass. Does it seem to "float" at the base? Or is it firmly anchored, resisting moving it? If it seems to be floating rather than being anchored firmly, (which would point to a tumor) it's a cyst and cysts are easy to pop out by making a small incision at the very top. But I would try dislodging the horn first.
To make it easy, take a small rag rung out in hot water, put a squirt of dish detergent on it, and then hold it against the horn for about five minutes. This should dissolve the horn into a jelly that should then lift off.
Wipe the bump well with alcohol and make your incision at the top, about a quarter inch is all you need. Then place two fingers at the base and slowly exert upward pressure. Yellow waxy pus should begin to extrude. Keep pushing upward until it all extrudes. There may be a pin feather in there that has looped itself back toward the base. If you see that, free it up so it springs upward. Use a toothpick. That would be what caused this monstrosity. Wash well with soap and water, and put a wound ointment such as Neosporin on it until the incision heals.
