From RWP Wiki
Yum-Yum is a baddy in the Banjo-Kazooie series.
Yum-Yums are large oysters with googly eyes. Their name comes from their enormous appetite.
In the Games
In Banjo-Kazooie, Yum-Yums are found in Treasure Trove Cove. While most attacks will do away with them easily, Yum-Yums are a particular nuisance because, if they manage to attack Banjo, they will force him to drop a Blue Egg and a Red Feather, in addition to losing health. If the items are not recollected quickly, the Yum-Yums will swallow them. They attack Banjo by bouncing into him.
Banjo-Kazooie: Grunty's Revenge
In Banjo-Kazooie: Grunty's Revenge, Yum-Yums are found in Breegull Beach and Spiller's Harbor. They act mostly the same as in Banjo-Kazooie, although they now cause Banjo to drop a Gold Feather when he is hit as well.