From the Super Mario Wiki
Incoming Chomps are a variety of Chomp only appearing in Super Mario World 2: Yoshi's Island and Yoshi's Island DS. They jump from the background hills and destroy everything below them. They also seem to be unlimited in number or they return to the hill after jumping. They only appear in certain levels, in areas marked with signs depicting them. Yoshi doesn't lose Baby Mario when hit by one of them. Chomp Rocks, which are stones in the shape of Incoming Chomps, also appear. In Yoshi's Island DS, a smaller variation called a Bound Wanwan appears, which bounces around the stage attempting to smash Yoshi.
The appearance of these enemies is different between the original SNES game and Yoshi's Island: Super Mario Advance 3, in that the reflective shine is no longer on both sides, and their teeth are shaded on the other side. This is actually made from graphics that were present, but unused in the original, likely to save on active sprite tiles due to the amount renderable at a time being limited.
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