Gold Ghost

The Gold Ghost is the first ghost Luigi encounters in Luigi's Mansion, in the Parlor, at first when he first gets the key from a 'gold spirit' in the Foyer, then after blowing all six candles out with the Poltergust 3000. The paintings in the room would come to life, then force fields would block the doors and three of these ghouls would appear, one-by-one. These ghosts would only have ten HP and would be easy to beat once Luigi was accustomed to his new vacuum. They would reappear countless times in the mansion.

Fire Ghost
Ghosts similar in appearance to the Gold Ghost but have fire inside of it would appear in the 2nd floor Washroom and the Sitting Room. In order to expose their ten-HP hearts, Luigi would have to find a Water Elemental Ghost and spit out water from his Poltergust to douse the fire.

Trivia
Incidentally, ten Gold Ghosts are used for practice at Professor E. Gadd's labratory.