Monsters I like but haven’t used yet:
Belkir
Asthma-inducing smoke elementals that like to stick their claws down your throat and slash your lungs to ribbons from the inside.
In Pathfinder 1e, they resembled bat-winged beetles.
Drolem
A golem created from the corpses of dragons. I’ve incorporated them into my take on Eberron (Aundair uses them as tanks), but my players haven’t fought one yet.
Grootslang
A giant snake-elephant thing.
Neh-Thalggu (Brain Collector)
Something like a giant crab with a row of brains on the top of its shell.
I see them as Deathclaws to the Illithids’ Enclave (if you’ve ever played Fallout 2, then you should know what I’m talking about).
Sahuagin
Okay, first off, I’ve fought these things as a player. I think they are awesome...but I’ll never ever use them as a DM.
Nothing I do with the Sea Devils will ever top what my dad did.
You see, my dad envisioned the sahuagin as humanoid hyper-carnivorous salmon, and even years later, everyone involved in that campaign still refers to it as “the Invasion of the Horny Fish-Men”.
Vegepygmy
Haven’t used these guys yet, but I’m currently building a homebrew setting where they will play an important role.