Fistula

Fistula is a boss that can appear in the Cellar. The large size Fistula has 60 HP, the medium forms have 15 HP, and the small forms have 8 HP.

Behavior

Moves around the room slowly, bouncing off the walls.
When enough health is depleted, the one large Fistula breaks into four medium Fistulas.
Each of the four medium Fistulas breaks into two small Fistulas.
After the smallest Fistulas are defeated, they spawn Chargers unless they are over a pit or other obstacle.
All stages of Fistula will do 1 red heart of contact damage.

Dark Grey: 15% larger with 25% more HP (75). Spawns Boom Flies instead of Chargers. Each piece travels much slower and splits into three medium Fistulas as opposed to the original's four, similar to Teratoma.

Fistula appeared in the original The Binding of Isaac.
A fistula is an abnormal connection between organs, intestines, and other tissues.
Fistula’s splittings after taking enough damage is most likely a reference to the game Asteroid, where asteroids would float diagonally and split after taking enough damage. This was confirmed by Edmund McMillen.
Fistula used to be a Caves/Catacombs boss in the original "Binding of Isaac", but as of Rebirth, it is a Cellar boss.
Teratoma is the posthumous version of Fistula.
Unlike the original "Binding Of Isaac", Fistula doesn't bounce off of Rocks.
Fistula was inspired by a boss of the same name in another Edmund McMillen game, C-Word.
Fistuloid is an enemy variant of this boss combined with what looks to be a Charger or Spitty.
The Matriarch is a boss added in Booster Pack #5 that is simultaneously based of a Fistuloid, Chub, and this boss. The Matriarch will also spawn small Fistulas throughout the battle.