Kangaroos!
Carnivorous Kangaroos, with opposable thumbs and vocal mimicry.
Night Bees! With infrared sensors like snakes. And they are vampiric!
They swarm and paralyze and drain. Maybe not enough to kill, but horrible shit to run into.
Just run away.
If you manage to find and capture a nest without destroying it you find "Night Bee Honey", which is made from blood. Specially preserved just like honey. (Imagine if blood banks had a way to store blood like that.)
The Night Bee idea is mine, but the Blood Honey is from a defunct UnknownArmies page. It involved ritualistically hanging a guy from meat hook and growing a colony of fly larvae in his ribcage. They would produce "Fly Honey".
fake edit:
WOW! I didn't know about this stuff!
Polymorph spells like in Nethack.
Even make a clone technique where you can transform to different sex and collect "sample" then become woman and self fertilize.
With some sort of Animal Growth spell you can make your own army of mini-me.
lots of little things like that
Please do not pick the hackneyed D&D shit.
No drow, beholders, dragons, mind flayers, derro, etc.
No elves, dwarves, gnomes, halflings (unless you make them genetically engineered circus midgets), etc.
Maybe have the flying squirrel things (
phanaton)
I remember those from Isle Of Dread. Maybe make them playable.
Various insect monsters, oozes, jellies, plants, etc. maybe Flumph.
Those mushroom guys are great. I always like those. You could give their poison an effect like getting drunk from the fountains in Telengard.
For uber-bosses have just one of a kind things that can exist only once per game: a Single Undead Beholder, a Single Dragon Turtle, a Single Aboleth, and a Single i dunno.
These ideally should not even be enemies- at least not the player's enemies. Maybe they entertain themselves by plotting against each other in various ways. Each one could have different victory conditions selected from a list at the beginning of each world-creation. So you could worship/get quests from them. This would add a semi-random meta element to the game.
I forgot: Vegepygmies!
Some of the old Gamma World creatures might be ok. At least the Obb.
Gotta have Obbs.
They would give you the strangest quests.