Dusanu
The Dusanu is an old and obscure Gotcha Monster from Dungeons & Dragons. Whilst it at first appears to be a humble skeleton, it is actually a parasitic fungus that feeds on dead flesh and which can, somehow, infest bones and then animate them to use them as a way to find new sources of food to eat and germinate spores on. As a result it's actually not hurt by anything specifically focused on the Undead, it's actually impervious to the trusty bludgeoning weapons you would normally rely on to destroy a skeleton (the bones have been left all rubbery, so they just sort of bounce back into shape), and it can deliver a very nasty rotting disease slash mold infestation with its claws.
The Dusanu appeared in the adventure module X5: The Temple of Death, and was converted to BECMI in the Creature Catalogue. It was updated to Advanced Dungeons & Dragons 2nd edition in the Mystara appendix for the Monstrous Compendium. It appeared in Dungeons & Dragons 4th Edition in an article in Dungeon Magazine.