Random memorable monsters that have only appeared once:
Avalancher (Monster Manual III, 3.5)
Monster that creates avalanches to kill passers-by so it can feed on their corpses. Just seems mean.
Crawling head (Fiend Folio, 3.0)
What's scarier than a crawling claw? A giant crawling zombie head!
Egarus (Planescape Monstrous Compendium Appendix III, 2E)
Fungus from the Abyss that made it to a world in the Material Plane, threatened to wipe out everything on the planet, and was only stopped by exile to the Quasielemental Plane of Vacuum... and adapted to survive there, too. I don't know if I'd ever use this in a game, but I love the lore here.
Facet (Planescape Monstrous Compendium Appendix III, 2E)
Creatures from the Quasielemental Plane of Salt that want to invade the Plane of Water. Also, up to five can combine together into a bigger, more powerful facet (which my brain incorrectly visualizes like something from Transformers). Shame the Quasielemental Planes don't get any love in 5E.
Hypnosnake (Creature Catalogue, BECMI)
Technically this first appeared in
Curse of Xanathon, and was reprinted in the Creature Catalog, but since the latter is just a revised reprint and the former is an adventure, I'm counting this as a one-off. Anyway, I think this one only stuck with me because of an appearance in the Spellfire card game back in the 1990s. But I'm surprised "hypnotizing snake" hasn't been in D&D more often.
Infernal (Epic Level Handbook, 3.0)
An abomination created when a god and a fiend have a baby. I feel like there's a lot of untapped potential in this idea.
Oard (Creature Catalogue, BECMI)
Technically this first appeared in an adventure (
Where Chaos Reigns); also, they were dropped from the Creature Catalog update. Anyway, they're cyborgs from the future, very Borg in design (but without assimilation). Admittedly, if you aren't cool with science fiction in your fantasy, these probably won't appeal.
Tiger fly (Fiend Folio, 1E)
I think I only like them because they look like something from The Outer Limits. But even the folks behind the Tome of Horrors didn't think they were worth converting...