Corpse Eater

Corpse Eaters are monstrous enemies that dwell in the later chapters. Their behavior is similar to that of a Grub, wandering around the room aimlessly and charging at the player if he gets in its line of sight. Greater caution is advised, however, as Corpse Eaters move a lot faster than Grubs. Also, Corpse Eaters will occasionally target enemies such as Small Maggots, Gapers, or even bosses like the Gurglings, and eat them. Upon successfully doing so, the Corpse Eater will gain health (based on the health of the eaten enemy, up to a maximum of double its original health) and start spitting out large quantities of blood shots and bones in all directions as they move about, creating additional hazards.

Carrion Rider
The Carrion Rider is a rare variant of the Corpse Eater. It is a Bony riding a Corpse Eater, making it one of the only fusion enemies in the game. It behaves similarly to the Corpse Eater, while the Bony riding it rapidly fires bone projectiles at the player. Upon defeating it, the Corpse Eater dies, leaving a now unmounted Bony left to attack the player on foot.

Trivia

 * If a Corpse Eater eats an enemy, its mouth will turn bloody. They'll also leave a puddle of creep until their mouth stops being bloody.
 * The Carrion Rider was possibly inspired by Minecraft's Spider Jockey, an enemy fusion consisting of a skeleton riding a giant spider.