I recently watched the Simpsons Treehouse of Horror episode in which they did a parody of "The Island of Dr Moreau" and thought a mad scientist trying to turn people into animals could make for a fun minor character in my campaign. My players are en route to a dungeon and should be there in a few sessions, I'm thinking of making the dungeon the lab of a mad scientist who has gotten locked out of the lower levels due to a containment breach.
The details I have so far is that he is a gnome, currently named Prof. Moreau, who has created mongrelfolk while trying to turn a human into an animal.
The mongrelfolk are safely contained in the lower levels and have started to create their own society, I think the boss of the dungeon should be a Gibbering Mouther called One, as in Attempt One.
I think the Mongrelfolk should worship one as their leader and hate Moreau for keeping them locked in the lab.
What I'm stuck on now is what life is like for the mongrelfolk, what they believe and how they have organised society.
If he's experimenting on humans, and depending on how much of their minds are retained from however he's transforming people; it could be that some of them recall bits of their previous religions and have bastardized them together with The Professor being the central antagonistic deity responsible for all their woes, and the gibbering horror as a living martyr for giving up of its sanity in order to banish the professor from the lab.
The lab could even be seen as a sort of corrupted garden of eden.
Bonus points for art and holy texts or statues depicting the professor as a faceless, looming, menacing shadow in a white coat.