Polterpup

Polterpups are white ghost dogs that appear in Luigi's Mansion 2. These dogs take various objects of importance that Luigi needs, ranging from Keys to Gold Dog Bones. A Polterpup may also revive Luigi when he loses all of his HP if he happens to have a Gold Dog Bone on him. Polterpups have 50 HP.

The Description in E. Gadd's vault says:

????

"I haven't come up with a name for this mischevious and adorable Polterpup yet. He's caused us a whole mess of trouble, but I think the playful tyke just wants someone to take him in."

The official website states, as well as the above:

"These playful canines have a strange and voracious appetite. They tend to eat objects you need, running off with them at exactly the wrong moment."

Luigi takes ???? in and adopts it at the end of the game.

Trivia

 * When a Polterpup passes through a wall it leaves a imprint behind, similar to Slimer from Ghostbusters.
 * "Polterpup" is a portmanteau of "Poltergeist" and "Puppy". Thus giving it the name, "Polterpup".