No skeleton
The monster doesn't leave behind a skeleton when its corpse rots away.
Effects
Casting Animate Skeleton has no effect on the corpses of these monsters. This means that if you're using that spell as an ersatz means of butchering, you won't be able to eat these corpses. There are usually enough corpses with skeletons that this doesn't matter, but it can be a factor in branches such as the Spider's Nest.