From the Super Mario Wiki
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Sky Guys are Shy Guys that float using balloons. They appear in Paper Mario, where they shoot at Mario with a slingshot. Sky Guys are always red. The balloons keeping them afloat can be attacked, grounding them and turning them into normal Shy Guys.
Outside of battle, Sky Guys attempt to swoop down at Mario to try to make the First Strike just like Paragloombas. They attack like Spy Guys but with their Hammer attack as well and are in camouflage gear.