A question for modders/ Modding and new creatures

I dont know a thing about modding myself. But I wanted to ask a question. How hard is it to make new creatures for the game. I have long thought that some of the old standards in dungeon critters would be nice to have. Examples would be rust monster(eating nice metal weapons), slithering trackers to interrupt a nice rest, piercers to drop down on a party from above, and trappers to come up from the floor.
Myconids were a great addition I thought to the original games. We already have some nice slimes and oozes. (can you detect my gray druid tendancies yet, hehheh).
Just thinking aloud here and wondering what you folks thought.