To help differentiate bone devils from being boring popcorn monsters with no special attacks or abilities, give them the ability to summon skeletons.

This should come with reasonable limits, such as setting their mspec_used field so that they can only do it every so often, and preventing them from doing it at all if there are a certain number of skeletons around them or on the whole level. Possibly limit it so that if they are not within the “misc item use” distance from the player, they will limit themselves to creating 0 or 1 skeletons.