Originally posted by Zep Tepi:

Bonemass spawns in the air and drops too. Someone else built a roof over the altar to keep rain out and BM got stuck up there. I think bosses normally spawn that way but we don't always catch them in the act. I have seen normal deer dropping while spawning in occasionally too. Probably prevents them from accidentally spawning under or inside the terrain.

The boars/deer spawning in the air is pretty funny, that being said I've never seen it with the first 2 bosses. I'll pay more attention next time.