Mega Maw

Mega Maw is a boss that can appear:

Behavior

Mega Maw is stationary and will perform the following attacks:

Fires red projectiles at Isaac in disorganized bursts, similar to Monstro.
Releases 2 rings of 5 flames which expand outwards and rotate in opposite directions.
Releases one large ring of 9 flames that expands outwards.
Releases one large ring of 9 flames that can rotate in either direction while expanding outwards.
Gradually spits out a Hopper directly below it.

Black: Spits 1-2

Spiders instead of Hoppers. The Spiders will be tossed directly towards Isaac instead of landing below Mega Maw. It does not use flame attacks.

Red: Doesn't shoot any flames or spawn monsters and has a completely different set of blood-shot attacks:

Fires a ring of 9 blood shots that spiral outward.

Fires 4-5 lines of 4 curving blood shots.

Fires 4 lines of 7 waving blood shots.

Fires 10 lines of 4 blood shots in all directions.

All the attacks of the Red version do 1 heart of damage.

Mega Maw in-game.
Black Champion Mega Maw.
Red Champion Mega Maw.
Mega Maw as one of Delirium's forms.
Concept art for Mega Maw (known in this sketch as "Mr. Maw Mega"), along with a scrapped variant of Pin and what is presumably an earlier version of The Gate.

It is the boss version of the Maws.
The Gate is the posthumous version of it.
Before its fire attack, a purple flame can be seen emanating from its forehead.
Despite its name, it acts and looks more like a Horf (or a Psychic Horf) than a Maw, because it does not move and its eyes are more round.
Its fire attack could be a reference to Mega Man's leaf shield/attack.
The visual appearance of the Red Champion version may be a reference to the 2Spooky item.