Guts do not directly attack Isaac. Instead, they attach themselves to any environmental object, such as Spikes, Rocks, or even walls, and start rolling around them. They are commonly found in groups of two to six, and can damage Isaac easily if the player is not careful.
Mama Guts is a new enemy added in the expansion Wrath of the Lamb expansion and can be found in Necropolis and Utero. It appears as an large blob of internal organs, much larger than regular Guts. Instead of attaching to an object, they slowly move around the room, eventually firing 8 large blood shots. Killing it will cause it to split into two Guts.
This enemy functions identically to the MemBrain.