- There are several which are involved with side quests including Old Blue Fin (a unique, named Slaughterfish,) a Giant Bull Netch, The White Guar (and several named pack guars,) The Dreugh Warlord.
- One particularly notable example is Beldoh the Undying, a unique named skeleton found in the very late-game dungeon Vemynal, within Red Mountain. He seems very out of place in a Sixth House citadel surrounded mostly by Ash creatures and Corprus beasts, and is in fact one of the weakest enemies in the area he is in. Still, he carries one of only two Blood Feat shields in the game. He's not related to any quest and there is no mention of him or his backstory anywhere else in the game.
- The Worm Lord is a powerful skeleton that can be found in his tomb in the Urshilaku Burial Caverns. While one does need to visit the caverns as part of the main quest, the Worm Lord is in his own section which is completely optional to visit. He's a relatively tough foe and guards some decent loot, but he has no known backstory. He is also notable for being one of the few non-humanoid enemies in the game that cannot be soul trapped.
- Oblivion has the unicorn, the giant mudcrab, the giant slaughterfish, and the painted trolls who inhabit their own unique little pocket dimension that looks nothing like the rest of the game.
- Skyrim features a couple examples of enemies unique to their dungeon/quest. One is the Afflicted, the vomit-spewing worshippers of Peryite. (Afflicted may randomly appear in the wild, so to speak, but they're extremely rare and their obvious purpose is to guide you to said sidequest.) Another example, made stranger by the fact that there's no clear in-story explanation, is the Corrupted Shades in the quest The Break of Dawn. They were explicitly created by a necromancer, but he's far from the only necromancer in the game, and all others use either ghosts, zombies, or Draugr. The Doylist explanation might be that the quest is given to you by Meridia, a Daedric Prince who despises necromancy and the undead, and if the player fought more standard enemies (that didn't collapse into puddles upon death as the Shades do), they could themselves use necromancy within the quest.
Unique Enemy / The Elder Scrolls