Tumor

Tumors are enemies that travel across the room diagonally and have the ability to float. They can randomly fire 6 blood shots in all directions, and fire even more quickly if getting hit.

Psy Tumor
Psy Tumors are pale versions of Tumors that show up on later levels like, and the. They behave exactly like regular Tumors, but fire homing shots instead of blood shots.

Camillo Jr.
Camillo Jr. is a variation of Tumor that appears in and the. They are colored black and have an eye in their center. They move exactly like Tumors, but fire lasers at Isaac. Their lasers may also hit enemies for 8 damage.

Planetoid
The Planetoid is a variant of Tumor that was introduced in The Binding of Isaac: Repentance. They move faster than tumors and, rather than firing radial bursts of blood shots, fire blood shots that orbit them, similarly to Isaac's tears when he has. When they take damage, they fire shots more frequently and have a chance to fart, firing their ring of blood shots outward and starting a new ring.

Poofer
The Poofer is a variant of Tumor that was introduced in The Binding of Isaac: Repentance, and behaves very differently from other tumors. They slowly float toward Isaac and, after taking damage, swell up and fire 6 blood shots in a fixed, radial spread. It swells up a total of 4 times and the more swollen a Poofer the slower it moves. Upon death, it explodes with a wide blood blast, healing nearby enemies for 15 HP, forcibly pushing Isaac, and leaves a large puddle of damaging Red Creep. Their health also doesn't deplete until they are hit by a total of at least 40 damage, giving them an effective health of 60 and a unique form of one-shot protection.

Swarmer
Swarmers are a variant of Tumors but does not fire projectiles and instead spawns fly monsters. It will occasionally spawn Attack Flies while floating, when hit it will spawn a Pooter, and when destroyed it will spawn a Boom Fly. Unlike other Tumors, Swarmers do not deal contact damage.

Trivia

 * Tumors are a reference to abnormal growth of tissues of the same name, caused by genetic mutations.
 * Camillo Jr.'s name is a reference to Kracko Jr., a recurring enemy from the Kirby series. In appearance he bears resemblance to Kracko, another recurring enemy from the Kirby series.
 * Swarmers are the next and possibly the final stage of the original Gaper.
 * The ring of Flies that surround the Swarmer is an indication of how much health it has. The Flies deplete as the Swarmer takes more damage.
 * Poofer is a reference to the enemy from The Legend of Bumbo with the same name.
 * A Planetoid is an astronomical object that is neither a planet nor a comet.