From the Super Mario Wiki
Fleepers are enemies from Yoshi's Island DS that are reminiscent of the Frog Pirates from Super Mario World 2: Yoshi's Island, but look and act differently. Two immediately noticeable differences is that they're yellow, and look more like fleas than actual frogs (which is probably how they got their names). They also bear a passing resemblance to a prairie chicken. Additionally, instead of lashing out with their tongues in an attempt to steal the baby that the Yoshi is carrying, they attack by trying to jump on top of him. With their long and springy hind legs, they can jump up into the top screen. They can be defeated by jumping on them or throwing an egg at them.