Fly Guy R thieves
From the Super Mario Wiki
The Fly Guy R thieves are a group of Fly Guys R fought as mini bosses during Mario and Luigi's hunt for the Ultibed parts, specifically the Driftwood Jellyfish Sheets. They are the only bosses in Mario & Luigi: Dream Team that are identical in both abilities and appearance to other common enemies.
When Mario and Luigi enter the hidden cave on Driftwood Shore to find the Driftwood Jellyfish Sheets, the Fly Guy R thieves are heard talking about how they plan to steal the treasure from inside. When Mario and Luigi actually reach the Jellyfish Sheets in the cave, the thieves appear and steal them, running away and knocking over the Mole Hunt proprietor nearby in the process. The Bros. are asked to play Mole Hunt a second time, which they do so for whereabouts of the thieves.
They are found at Pi'illo Blimport, where the party come up with a plan to distract them and get back the sheets. Prince Dreambert and Starlow loudly talk about a "rumor" involving a birthmark on Bowser, while Mario and Luigi creep around and steal the sheets back from behind the thieves' backs. However, the Fly Guy R thieves notice the Bros. when they get the Jellyfish Sheets, attacking them. Once they are defeated, they do not appear for the remainder of the game.