From the Super Mario Wiki
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Sentinels are small flying creatures that act as guards in Tubba Blubba's Castle. In Paper Mario, Mario cannot fight them, and they cannot hurt him. When Mario gets too close to a Sentinel, it starts to flash and tries to grab him. Lady Bow can use her Outta Sight move to turn her and Mario transparent to confuse the Sentinel. If one catches Mario, it throws him and his party out of the castle.
When Tubba Blubba chases Mario and crashes the bridges above the main room, the Sentinels are crushed by the debris. They disappear after the chapter is over.