Boo Blah

A Boo Blah is a kind of ghost that appears in Super Mario World 2: Yoshi's Island, its remake Yoshi's Island: Super Mario Advance 3 and Yoshi's New Island. This creature sticks to the ceiling or the floor and stretches out periodically, similarly to Stretches from Super Mario Bros. 3. They are generally invincible though they only hurt the player if they make contact with them while they are extended. Stomping them stuns them for a brief period of time and by holding the jump button, Yoshi can bounce off them. They can be defeated by using a POW Block or a cloud item. Also, there is an unused variant that has a Piro Dangle on its head, which was used in an earlier build of the game.

Colored Boo Blahs appear in both the later versions of The Legend of Zelda: Link's Awakening in the optional Color Dungeon. For The Legend of Zelda: Link's Awakening DX, they are localized as Camo Goblins, and for the Nintendo Switch remake, they are localized as Goo Specters. They appear in red, green, and blue versions and disguise in their small forms against same-colored floors, pursuing Link before stretching out.

Super Mario World 2: Yoshi's Island

 * Official Nintendo Player's Guide: Lurking in the stonework of Bigger Boo's castle, the Boo Blah's "Boo!" is worse than its bite.