Gaper

Blurb
The Blurb walks slowly towards Isaac and fires a chaotic burst of tears if they get close or take enough damage. After firing their tears, they act like regular Gapers and chase Isaac faster than before, attempting to deal contact damage. In the fight against Mausoleum Mom's Heart, bloody versions of Blurbs with pinker skin can be summoned during the battle.

Tidal Gaper
The Tidal Gaper pops out of the water and chases Isaac when all other enemies in the room are killed. It has a chance to spawn with a Small Leech attached, which will detach when the Gaper is killed. Some Tidal Gapers spawn with seaweed on them, but they do not act differently.

Wraith
The Wraith chases Isaac. It is detectable only by its reflection and the ripples in the water, and the occasional lightning that makes them briefly visible.

Danny
The Danny walks towards Isaac, periodically firing several rocks towards him, some of which may be Rock Spiders.

Coal Boy
The Coal Boy walks towards Isaac, periodically firing several hot rocks towards Isaac, some of which may be Coal Spiders. Hot coals linger and can hurt Isaac.

Faceless
The Faceless wanders aimlessly and fires tears in six directions. If Isaac gets close or damages a Faceless, it will start chasing him while firing tears more frequently.

Fanatic
The Fanatic is a cultist which appears exclusively in the boss fight against the Heretic. At the beginning they are seen in a group of 5 on the edges of a large purple pentagram on the floor, chanting and kneeling towards the center as the Heretic emerges. They are covered with shields and can not be killed until the Heretic is done spawning. Once they are done, they will chase Isaac while shooting homing tears periodically. Upon death, they have a chance to turn into Pacers or Gushers like normal Gapers.

Trivia

 * Fanatics resemble Exorcists when summoning the Heretic.

Strifer
The Strifer walks horizontally or vertically, attempting to line up with Isaac, and frequently fires lines of four tears forwards. In some rooms, the direction a Strifer faces is determined by which side of the room Isaac enters from.

Rotten Gaper
The Rotten Gaper chases Isaac, and may explode into Small Maggots upon death.

Twitchy
The Twitchy alternates between chasing Isaac and leaping at him. Usually, they rapidly leap 3 times, then return to chasing Isaac on foot for a few seconds.