Buttlicker

Buttlickers are monsters that wander around randomly in straight lines similar to Larry Jr.. They most often spawn attached to other Buttlickers in chains of up to 5. When one Buttlicker in the chain is destroyed, the other Buttlickers separate from each other and continue to move in random directions.

Similar to Larry Jr., Buttlickers can destroy Poop on contact. Often, Buttlickers are seen in rooms filled with Corny Poop, leading to swarms of Flies.

Butt Slicker
The Butt Slicker is an enemy added in The Binding of Isaac: Repentance. Individually they act identically to Buttlickers, but unlike their counterparts they can group together and stick to other Butt Slickers. When grouped together they begin to drag themselves towards Isaac. Upon killing a Butt Slicker within a group, it creates a pool of slowing Black Creep and shoots out black projectiles; the amount of projectiles released and the size of the pool of Black Creep is determined by the amount of Butt Slickers attached. All other Butt Slickers will scatter and temporarily leave trails of Black Creep when this occurs.

Killing a Butt Slicker in a group of 2 or 3 will create a small pool of Black Creep and 4 projectiles in the cardinal directions while killing one in a group of 4 or more creates a large pool of Black Creep and 8 projectiles in the cardinal and diagonal directions.

Trivia

 * Buttlickers were first implemented in the Wrath of the Lamb DLC for the original The Binding of Isaac.
 * In the original The Binding of Isaac, destroying any Buttlicker in a chain would cause all Buttlickers to separate and move independently. This does not happen in Rebirth.