Hell, I don't even remember where I can find Stantler. It is the kind of Pokémon that really deserves a mega evolution. I remember how badass they were in the anime.
Probably Unown, I used it like once when Gen 2 was first released because I was curious about it but when I found out it would never learn anything else than Hidden Power I dropped it completely. I also really can't imagine it getting used much at all in general with it not being able to learn anything and have crummy stats too.
There are so many I didn’t use but out of them all is Magikarp I already got a Gyrados because of the red shiny one so why even try to get a normal one with a Magikarp.
Unknown in general is one of the least used Pokémon in general, not just in Gen 2. I generally try to have at least 1 odd of uncommon Pokémon on my normal playthroughs just for the heck of it. I've actually really enjoyed using Pokémon like Stantler (decent stats, Intimidate, and decent movepool since it can use both physical and special moves with decent success), Girafarig (similar to Stantler in usefulness but Stantler has a better ability), Jumpluff (Super annoying with Leech Seed, Sleep Powder/Toxic, Bounce, Attract/Substitute/Giga Drain), etc. That being said I've also tried my best to make some underused Pokémon work but they struggle even in game >.> as Bloom... @Bubbles said Delibird is pretty trash. Bad stats, no lvl up moves, horrible typing. Yanma is gen 2 is pretty bad as well and vastly inferior to Scyther...I mean it doesn't even learn a Bug move in GSC >.> in the 4th gen remakes it becomes viable thanks to Bug Buzz and the potential to evolve. Ditto is also always kinda meh to me. I mean sure it can become any Pokémon and gains the same attacks as the Pokémon it transforms into but I simply don't like using it in battle.
Magcargo is one of those Pokémon that comes ot mind, alongside Stantler as many others have mentioned. Unown is basically kind of a gimmick Pokémon and I don't think it was ever meant to be used seriously in battle, but I guess it counts. I'm honestly unsure if Sudowoodo counts as unused but it certainly doesn't seem to be a popular choice either from the best of my knowledge.
Meganium, because Meganium is trash. I don't see Ursaring get much usage, if ever. Even Dunsparce and Unown ges more gameplay because of the memes. Ursaring is just kinda... there.