The Borewyrms are the other models in the Dreaded Ambull expansion for Blackstone Fortress. They’re basically baby Ambulls.
Especially interesting is what look like wings, suggesting that the Ambull larva can fly. But that doesn’t really have a place in the rules.
Like the Ambull, these models could fit in a fantasy setting, maybe some of the weirder monsters deep in the dungeon or baby Carrion Crawlers.