I think that inventing fan-lore for Gen 1 glitch-pokemon is easy, the whole thing practically writes itself.
Entities from a source incompatible with existence-as-we-know-it (some data which isn't a pokemon getting misinterpreted by the game's code as a pokemon), whose forms cannot be comprehended or properly perceived (glitchy static/garbage data for a sprite) or which appear to be something familiar, yet in a manner and context which is viscerally wrong (the ones which do appear as existing ingame sprites), whose names are unpronounceable (like "h POKé [sequence of distorted sprites]" or "▙ ♀ .") and/or are just epithets ("Missing Number"), and whose very presence is hostile to reality-as-we-know-it (causing the some or all of the game to glitch out).
Sounds like your regulation cosmic-horror eldritch monstrosities to me!
