The Great Potato is the holiday king of Leprechanuka. It comes to people who believe in it and make good deeds. His lore seems to be a combination of many fairy tales.
The Great Potato is the holiday mascot of Leprechanuka, and is known for helping the needy and gifting chocolate coins. In Leprechanuka, he asks his mirror if he is the fairiest of all potatoes, and if he isn't the hotter looking potato gets a poisonous apple, much like in Snow White. He also mentions that he turns back into a pumpkin after Leprechanuka.
It is a potato, and wears a black belt with a gold buckle, black framed glasses, green shoes, a green hat, and white gloves.
His human flesh colored ears and nose, not to mention white gloves, may make him a reference to the longtime popular children's toy, Mr. Potato Head.
Cosmo spoke about the Great Potato in the episode, Crocker of Gold. Although Cosmo was the only one who believed in him, when they were being cooked in a cauldron by the McPunchies, he showed up and burned the McPunchies for cooking people on Leprechanuka, and freed them.
His backstory and lore seems to reference many different things.
His name references "It's the Great Pumpkin, Charlie Brown.
The part with the mirror and poison apple references Snow White and the Seven Dwarves.
Turning back into a pumpkin after Leprechanuka references Cinderella.