Polterguy

Polterguys, originally called Phantom Guys, are ghost Shy Guys found in the Mario vs. Donkey Kong series. They resemble and may even be related to Boo Guys.

Mario vs. Donkey Kong
Polterguys first appear as enemies in Mario vs. Donkey Kong. Like other enemies in the game, coming in contact with Mario will cause him to die and lose a life. Polterguys cannot be defeated, but they can be turned into blocks when Mario presses a button the same color as the ghosts. This will allow Mario to pass. Polterguys first appear in the Spooky House world.

Mario vs. Donkey Kong 2: March of the Minis
In Mario vs. Donkey Kong 2: March of the Minis, they appear in the Spooky Attic stage. They function like in the last game for the most part, though now they are enemies to the Mini Mario toys, who serve as the game's playable characters. Once again, they can be turned into blocks when a certain button is pressed. This technique can be used to solve puzzles on the stages they appear. In the Construction Zone, Polterguys can be used in the Spooky Attic and Special Kit 3 sets. The latter allows them to interact with the game's new Mini Donkey Kong toys.

Future Installments
Polterguys have not appeared in any games since Mario vs. Donkey Kong 2: March of the Minis, but a similar enemy known as Pyro Guy appears in Mario vs. Donkey Kong: Minis March Again!.

Trivia

 * Polterguy comes from the words "poltergeist" and "Shy Guy".