Originally posted by Renlish:

The bosses are tied to the seeds you beat them on. If you've killed the boss on one world, mounting the head on the rock in the new world won't count unless you actually kill the boss on that world too.

Many people have a world where they do all their active playing (fighting mobs/bosses) and a second for resources and building so they don't have to be bothered by the bigger, more annoying raids.

I haven't bothered to kill a boss twice, but if they drop their trophy again, you can take that to a new world to mount and it doesn't count to activate that level of raids? Interesting.