Here’s something I’ve been adding to sporadically: A collection of Monster Character Classes (along with atrocious art by yours truly!). These should be usable with BECMI or B/X (and the other assorted games that treat Demihumans as separate Classes) without much tinkering.
So far, we have the Arenea, Living Statue, Harpy, Gnoll, Tortoisefok, Flutterlings, and the Lizard Man. I have been tempted to go through one of my favorites, the venerable Creature Catalog, and craft some Classes for each of the stranger sentient beings in there.
Open to any suggestions of other Monsters that you might like to see given this sort of treatment!