2D pixel-art Pokemon-like where "glitch"-mons are canon.
Some of them are like "what would MissingNo look like if you had a mess of pixels in real life instead of garbage data dumped directly into tileset-graphics". Others look like Cthulhoid monstrosities or the weirder kinds of Ultra Beasts. Others, of course, would look like existing sprites you've seen before ... but Something Is Wrong.
They're normally inaccessible, but you can encounter them by setting off a Rube Goldberg chain of seemingly-glitchy nonsense. Depending on which specific ones you encounter, they can have various effects that look like the result of memory corruption in a Game Boy game, but which are actually happening on purpose. Basically, in-universe, they're eldritch monstrosities who are inimical to existence-as-we-know-it -- not because they're hostile or evil or whatever, but just as a side-effect of what kind of entities they are. Y'know, like the nastier kinds of Pokemon glitches.
If you catch one, this sets off a Glitch Route, where corruption starts spreading through the game world, resembling a cross between some kind of curse/disease/whatever turning it into a wasteland, and sprite-glitches. All the 'mons you fight there are the glitch types. The spread of the corruption is constrained by progress through the main route, and mostly doesn't affect the equivalent of Gym Badges and whatnot until after you've cleared them. Culminates in a "true final boss" thng against The Missing Number or some other kind of Ultimate Glitch, and beating it mostly undoes the glitching ... but there are still faint leftover traces here and there ...
