Cornimer (ドンどんぐり Dondonguri?, Don Acorn) is a tortoise that appears in the Animal Crossing series. He only comes out during the Acorn Festival in Wild World, and gives the player mushroom furniture, walls, and floors in exchange for acorns, which are obtained by looking under oak trees during the festival. However, if the player gives him a rotten acorn, he will say that all the other acorns the player gave him that time don't count. Much like Tortimer telling the player's fortune by using fireworks he can tell the player's fortune by using acorns, as shown below. It is confirmed through a bunch of obvious hints that Cornimer is Tortimer wearing a mask.
Cornimer's head appears on the machine that the player must chase and whack with a hammer in the Hammer Island Tours in New Leaf. It is possible that the head is the Cornimer mask allegedly worn by Tortimer in Wild World, attached to a remote control car.
He was later added to New Leaf full-time in the Welcome amiibo update. He is the final boss of the Puzzle League minigame that can be played on the New Nintendo 3DS furniture item, which even more brings confirmation that he's Tortimer due to multiple references you can connect to Tortimer, such as his speaking pattern.
Super Smash Bros. Brawl
In Super Smash Bros. Brawl, Cornimer is an obtainable sticker. He can also be occasionally be found on the Town and City stage.