Red Boos, also known as Pink Boos, are a pink or light red variant of Boos, and seem to be not much different as normal Boos, except for their speed and trickiness.
Possibly, they took over the Mario Party role of a normal Boo, as the white one was playable since Mario Party 5. They were able to steal Coins and Stars. The stealing mechanic was later deleted in further Mario Parties, and so they just appeared in Mini-Games. In Mario Party 8, they sold Candy in a particular board.
In "Mario and Bowser 2 War!" They with help from the normal boos posses Luigi and put him in a cage.
Red Boos make an appearance in the Mario Kart title, Mario Kart Silver. In the game, they appear as items that act exactly the same as the previous Boo items from other games in the series, in which they steal items from racers. Should, for whatever reason, no racer have an item, the Red Boo will disguise itself as a red, translucent Fake Item Box until someone does obtain one.