You should probably look up the Open Gaming Liscense (OGL) and what you can or cannot do with respect to it. Generally, classic fantasy monsters (Dragons, Manticores, Sasquatch, Vampires). Are fair use. OGL also allows for creatures that are similar to D&D exclusive monsters to exist so long as the name is changed sufficently.
Most "monsters" are in what's called Public Domain and are free to use and modify. Additionally "powers" of a monster (or superhero) aren't generally copyrighted but the totatllity of their use in a work can be (does your superhero fly? Is super-strong? Is invulerable? Can be fine. Is he named Clark Kent? That's a problem).
Fair Use also allows for some parody but again, it's a defense to copyright infringment and not a liscenses to take someone elses work wholesale. It also doesn't stop them from suing you, as you have to claim fair use as your defense if and when each suit arises.
I'd recomend looking at National Comics Publications, Inc. v. Fawcett Publications, Inc. for an example of an intellectual property dispute that is close to yours. Note that Fawcett won at trial but lost on appeal and rather than take the matter before SCOTUS decided to settle out of court.