Sheldon did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
“We’ve just confirmed that this horseshoe crab dude is the owner of the weapons shop. He’s reportedly obsessed with all types of military gear. This love has come at a cost, though, as cleaning and polishing his gear all day every day has apparently made him extremely nearsighted. No wonder his glasses are so thick. What a nerd!”
— Splatoon Official tumblr
Sheldon is a Horseshoe Crab character from Splatoon. He acts as one of the shopkeepers in the game, owning the store Ammo Knights and selling weapons to the Inklings. According to early press information, he is obsessed with military gear, and teaches the Inklings how to use the weapons. He is extremely nearsighted, having become this way due to the long hours he has spent polishing and cleaning his gear.