Peepa

Peepas are Boo enemies debuting in Super Mario 3D Land. They are small, white ghosts with stubby arms, black eyes with white pupils and a permanently smiling mouth which shows its two teeth.

Super Mario 3D Land
Peepas are exclusive to the Ghost Houses found in the game. These enemies are usually found alongside Boos. Peepas do not directly attack Mario or Luigi when they get too close, but rather stay in a stationary position or moving in a set area, making them more of an obstacle rather than an enemy. Sometimes, several Peepas encircle a select area similar to the Octoboos in Super Mario Galaxy 2. A Peepa can only be defeated by running into it while under the effects of a Starman.

New Super Mario Bros. 2
Peepas made their next appearance in New Super Mario Bros. 2, marking their first sidescroller appearance. They act like the Circling Boo Buddies of New Super Mario Bros. Wii and Super Mario World. Like most Boos of the game, they have a bluish complexion. Peepas first appear in World 2's Ghost House.

Also, in World 3's Ghost House and other Ghost House based levels, Peepas carry moving yellow platforms. They appear when Mario stands on one of them. They will move along with the platform tracks if there is any. Sometimes they will tilt the platforms and also they may disappear at the end of a track, causing the platforms to disappear as well. These platform-carrying Peepas do not hurt Mario or Luigi if they touch them, unlike the circling Peepas.

Super Mario 3D World
Peepas reappear in Super Mario 3D World. Unlike in previous games, they can be defeated by the light of a Light Box.

Mario Party: Island Tour
Peepas appear in Mario Party: Island Tour. In the minigame Get Reel, a few can be seen inhabiting the corridor, where they end up chasing Mario. They also appear in the minigames Horror Harriers and Peep a Peepa. The fact that Boo is a playable character makes it reasonable for why Peepas appear in minigames.

Super Mario-Kun
Peepa appears in volume 46 of Super Mario-Kun. He encourages a Tail Boo to scare Mario. At the end of the chapter, Peepa gets buried by an avalanche along with Tail Boo, Tail Boo's father, and the Bros.

Trivia

 * Peepas bear a resemblance to the Ghost Monsters in the Pac-Man series.
 * Peepa's face looks similar to the faces of the Circling Boo Buddies.