Mummy

No Spoilers
Mummy likely has the most lore in the game, the most anyone really knows is that he's the oldest animatronic in the game, dating all the way to 1985, it's a marvel he works to this day.

Spoilers Ahead
Mummy is an original, one of the ones from the old Freddy Fazbears pizza, if you collect all the newspapers in FDaH 1 you learn that the old "Bonnie" went missing, the reason he's called mummy, is because centuries of being T.Ped by teens over the years has took a toll on him, and his kind soul, this would mean he's the only child soul not set free.