From the Super Mario Wiki
Skeleton Goonies are skeletal undead versions of Goonies that appear in Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, Yoshi Touch & Go and Yoshi's Island DS. Unlike normal Goonies, they may not be ridden on, and will lose their wings and fall if this is attempted. They also fall if attacked with an egg. Some carry bombs to drop on Yoshi, similar to regular Goonies, but they never carry green Shy Guys. Oddly enough, they have feathers even though they have skeletal wings.
In Super Mario World 2: Yoshi's Island, even though they had their own level, they debuted in the first skiing part of World 5.
A flightless variety exists known as the Flightless Skeleton Goonies.
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