Discussion of the Necropolis faction.

*theoretically, a bone dragon (if it were here) would be a level 8 creature or a boss... because even a swordsman can damage weavers (as in the intro video), but a dragon and an axe won't do the trick...

Bone dragons exist. They are just considered neutral creatures, so it's difficult to recruit them on the map.
In Wayne's campaign, you can seemingly even recruit them in the castle, but I'm not sure.
And there's also the dynasty weapon "Belket's Staff" - it allows you to summon these dragons in every battle.
And there's also a boss bone dragon - it's in Sandro's campaign.