Stillborn

Stillborn is a strange, half-stone half-fleshy enemy located in the 4th chapters. They slowly float towards Isaac while periodically firing blood shots at him. They can not be killed, but upon clearing the room, the fleshy appendage that attaches them to the ceiling will break and they'll helplessly fall into the ground. They will then explode after a few seconds unless the player leaves the room quickly.

If a Stillborn is over a gap when the room gets cleared, it will immediately explode upon impacting the ground. However, if a Stillborn is over a rock when the room is cleared, it will destroy that rock upon landing in the ground.

Trivia

 * The word 'Stillborn' is a term given to a fetus that died during the process of birth.
 * In the Corpse chapter, all Stillborns have a rotting green coloration to their fleshy appendages.
 * Unlike most enemies, Stillborns do not seem to flip their sprites to face Isaac if he circles around them.
 * Stillborns are one of a few non-boss enemies to have death animations, the others being Death's Heads (and its variants) and Screamers.
 * Stillborns are inspired by Lithopedion, or stone baby, a rare phenomenon where a fetus dies during an abdominal pregnancy, stays in the body and calcifies.