Birthday Cake (お祝いケーキわんこ Oiwai Kēki Wanko, Celebration Cake Puppy) is an enemy that appears in the Birthday Present! stages.
This is actually a Metal enemy, 3 Doges inside a birthday cake. Cow Cat and any other fast attacking characters such as Mr. can kill this enemy very easily. It deals pretty much no damage per hit, so there's no need to worry about it at all.