D&D General Creatures Obscure & Underused

[Levistus's_Leviathan]

I think so. But ancient vanquished evils returning isn't exactly abnormal in fantasy.

Good point. It isn't surprising that they haven't been statted out recently, due to the fact that they're extinct.

 

[Azzy]

Next chance I get in a Forgotten Realms campaign I'm making players fight a Phaerimm. Weird as hell, important to the setting lore, and yet haven't been given stats for several editions; it's a damned shame. Their days as cover models shall return!

Sadly, that's the least comical depiction of the creature that I've see.

 

[Theo R Cwithin]

That phaerimm picture on the Netheril cover reminded me of the fihyr in the 3e MM2, a creature that coalesces from peoples' collected nightmares (called feyr in 2e). I haven't seen them used, but apparently they appeared in some of the D&D computer games.
The 3e rendition always made me want to use one to terrorize a neighborhood or something in an urban campaign.

Spoiler: monsters!

 

[Levistus's_Leviathan]

A few others:

Banshee. I don't know if I want a TPK, but this monster seems cool, and I would like to use it. Unfortunately, I haven't really ever found a situation to add them in.

Perytons. Strange monsters, with cool lore. Again, never found a situation to use them in.

Balhannoth. Again, never had a campaign in the Shadowfell, and have never felt the need to add them to a campaign, but in 5e they're absolutely awful and horrifying monsters.

 

[R_J_K75]

There's a creature that is basically a walking stony, spiky structure of holes and tubes. Wind blowing through the creature's body produces soft whistling noises that can pacify lesser undead.
I couldn't remember the name of it, so I had to poke around til I found it in the 1e Fiend Folio: sussurus.
(Because of course. .)

It's an odd beastie with an odd ability that's inspired a couple encounter ideas years ago, but I've never had a chance to use on in play.

It showed up in 2E somewhere too because I used it on a party but it was so long ago I dont remember it must not have made for a very memorable encounter. Oddly though I remembered its name halfway through your post.

 

[R_J_K75]

I think so. But ancient vanquished evils returning isn't exactly abnormal in fantasy.

Think they broke out of the Sharn Wall under Anauroch in the Return of the Shades novel series that came out early 2000s sometime.

EDIT: It was called the Return of the Archwizards trilogy.

 

[R_J_K75]

Banshee. I don't know if I want a TPK, but this monster seems cool, and I would like to use it. Unfortunately, I haven't really ever found a situation to add them in.

I used a banshee for the penultimate adventure for a campaign. The PC had to create a spell to defeat an undead horde. The spell component was the wail of of a banshee. When I told them they asked me how are we supposed to get that? I said I don't know you figure it out.

 

[R_J_K75]

I always wanted to use an Assassin Imp but it just seemed too evil. Basically the PC would go to sleep and never wake up. If it visited a few nights in a row you could have a TPK.

Imp, Assassin (Monstrous Manual)

 

[Voadam]

Morkoths. This is the opposite of Cyclopes for me. They're new to me, are cool and would be a great villain, but I have also never found a time in a campaign to add them in.

They first showed up in 0D&D's Blackmoor (which introduced a lot of aquatic monsters like the Sahuagin and the Ixitxachitl) then again in the 1e Monster Manual.

 

[Voadam]

Banshee. I don't know if I want a TPK, but this monster seems cool, and I would like to use it. Unfortunately, I haven't really ever found a situation to add them in.

They are in a lot of modules, most classically attending Lord Soth's court in Dragonlance and singing about his fall to evil each night. DL 16 World of Krynn comes to mind, but there are a ton of others. They are also often called groaning spirits in older editions. In Pathfinder they are a very strong hard hitting high CR monster. In AD&D they were very deadly for their save or die area wail. In Ravenloft there was a Domain Lord who was one.