Fly Guy R thieves

The Fly Guy R Thieves (also known as Fly Guy Rs) are a group of Fly Guy R found in Mario & Luigi: Dream Team. Fought as a mini boss midway through the game, they appear and steal the Jelly Sheets from Driftwood Shore when Mario and co approach, and are then fought at Pi'illo Blimport.

History
When Mario and the party enter the hidden cave, the Fly Guy R Thieves are heard talking about how they plan to steal the treasure from inside.

A bit later, when Mario and co actually reach the Jelly Sheets in the cave, the thieves appear and steal them, running away and knocking over the Mole Game running Shelltop nearby in the process.

They're found at Pi'illo Blimport, where the party come up with a plan to distract them and get back the sheets. Dreambert and Starlow talk to them about a 'rumour' involving Bowser, while Mario and Luigi steal the sheets back from behind their backs. However, the Fly Guy R Thieves soon notice the Bros., challenging them to a boss battle.

Once they're defeated, they never show up again.

Battle
In Battle, the Fly Guy R Thieves fight identically to the other Fly Guy Rs, summoning Goombas to attack or dropping them on Mario or Luigi to cause damage. They do have the Boss Theme playing in their battle though, as well as the special boss intro beforehand.

Trivia

 * The Fly Guy R Thieves are the only boss in the game that happen to be identical in both abilities and appearance to other common enemies.
 * They're also the only 'boss' characters not shown in the Hard Mode ending pictures.