Anything with horns (antelope, goat, bison,sheep, etc.) you'll need to boil the horns to get the horns off. There is meat/tissue between the horn/bone that you must remove.
I boil the bison horns for about 30 minutes. Take a knife and push it up into the edge of the horn/base to help get it started. I smack the horn with a bat to help release it. It can be hard to get bison horns off.