The Cage

The Cage is a boss that can appear:

Behavior

Leaps off-screen and tries to land in Isaac's path, predicting his movement. Leaves a small puddle of damaging Grey Creep and sends out 4-5 lines of rock waves upon landing.
Quickly shoots 6 streams of 3-7 projectiles in a spiral pattern, similar to Mom's Heart/ It Lives!.
Curls up in a ball and rolls towards Isaac, bouncing off walls and obstacles around the room.
Spawns a grey-coloured Vis that shoots a brown-coloured Brimstone laser.
- The different colours are purely cosmetic, and it behaves exactly like a normal Vis.
- Any Vis caught in The Cage's rolling attack will be killed.

Green: Periodically leaves puddles of Green Creep and doesn't spawn any Vis.

Pink: Two Cages, each with 50% HP (400), replace the normal Cage. They never perform their jump attack.

Both Cages are ~50% smaller.

The Cage in-game.
The Green Champion Cage.
The Pink Champion Cage.
The Cage as one of Delirium's forms.
Unused pre-release sprite for The Cage found among the Room Editor files.
Concept art of The Cage on the right side, along with other Rebirth boss concept arts.
Original Cage Vis sprite.
Cage Vis sprite in Repentance.

Host Hat and Pyromaniac make Isaac immune to The Cage's rock waves, since it is considered to be the same as explosions by the game.
When The Cage jumps, there is a delay to his falling animation, causing Isaac to be hurt before The Cage falls back down.
If Isaac is flying on Pits, The Cage can hit Isaac if it jumps towards him, and it will then be pushed back to the ground.

It has the highest health of any boss fought before Mom's Heart.
- Despite being a possible encounter before Mom, The Cage has more HP than her.
It is the posthumous version of Mega Fatty. It appears to have lost its head.
Although The Cage is a dead version of Mega Fatty, it also bears a lot of resemblances to a giant Vis as well, since it is a fat headless body with a giant cut in its belly. Another slight nod to their resemblance is The Cage's ability to spawn Vis with a Grey color palette and a Brown Brimstone laser.
The stitches on the Cage's body resemble Y-shaped incisions commonly made at the start of an autopsy.