From the Super Mario Wiki, the Mario encyclopedia
The Propellerpack is a backpack with a helicopter blade utilized by Baron K. Roolenstein to make him fly in Donkey Kong Country 3: Dixie Kong's Double Trouble!, Donkey Kong Land III, and Super Smash Bros. Ultimate. Its name is a pun on "propeller" and "backpack".
Donkey Kong Country 3: Dixie Kong's Double Trouble!
The Propellerpack first appears in Donkey Kong Country 3: Dixie Kong's Double Trouble!. In this game, Baron K. Roolenstein wears the Propellerpack as a backpack and uses it to fly around the levels Kastle KAOS and Knautilus. In both levels, the player must specifically hit the Propellerpack with Barrels and Steel Kegs, respectively, in order to damage Baron K. Roolenstein.
Donkey Kong Land III
The Propellerpack reappears in Donkey Kong Land III where Baron K. Roolenstein uses the Propellerpack to fly in the levels K Rool Duel and K Rools Last Stand.
Super Smash Bros. Ultimate
The Propellerpack reappears in Super Smash Bros. Ultimate as King K. Rool's up special. When used, King K. Rool equips the Propellerpack and flies upwards, where he can move horizontally. After a couple of seconds the Propelllerpack's blades slow down and he falls back downwards. When this happens, the player cannot dodge, attack, or jump until King K. Rool touches the ground, although they can still move horizontally.