Bonemass isn't a Jotun, but an abimination made of multiple dead bodies.
If you have a 2nd trophy, hang it at your wall and at some point you will see a message about thanking you or killing Bonemass, as it will release all those who were captured in it.

If you look closer to the trophy, you'll also notice it's made from multiple skeleton (-pieces), like two skulls for eyes, rib cages for teeth and so on.
So Bonemass isn't a Jotun at all.

The skull you summon him can be the remains of a Yotun, as later in the Mistlands you'll find ribcages and skulls as well as giant swords and helmets. The latter two are occasionally found near petrified bones. Not to mention, you find a similar kind of "soft tissue" in a petrified Jotunskill as you find inside a Dvergr skull.

It is however, likely that Yagluth was a Jotun when comparing the size of what's left of him with the petrified bones and skulls. It would at least explain why he was a King.

Thorin :)