Gaper

Gapers are a category of enemies that usually directly chase after the player. Revelations adds the following variants.

Frosty Gaper
Frosty Gapers are Gaper variants that spawn from Ice Hazards. Frosty Gapers act as regular Gapers, and will attempt to walk towards the player by taking the most direct route possible, dealing contact damage. Upon death they have a chance of leaving behind either a Frosty Pacer or a Frosty Gusher.

Ice Block Gaper
Ice Block Gapers act as regular Gapers and directly follow the player, dealing contact damage. When they die, their heads split off and slide away from the player. When the head collides with an object it shatters and releases the head as a Frosty Horf. Meanwhile the body will turn into either a Frosty Pacer or a Frosty Gusher.

There exist two variants of Ice Block Gapers, clumpy and cuboid. Both act roughly the same but with the main difference being the way their ice heads behave when knocked off. Clumpy Ice Block Gapers, distinguished by the more uneven even ice blocks engulfing their heads, can have their heads knocked off in any direction and when they shatter they release 4 tears in a diagonal x pattern. The heads of Cuboid Ice Block Gapers, based on their squarer shaped ice, will only slide in one of the cardinal directions and release 4 cardinal tears in a + pattern upon breaking.

Yellow Ice Block Gaper
Yellow Ice Block Gapers originally only appeared in the fight against champion Flurry Jr. but following Sin Update can be found in regular rooms within the Glacier. Yellow Ice Block Gapers are urine themed variants of Ice Block Gapers and behave roughly identical. However, when their heads get knocked off, instead of leaving a trail of slippery ice creep, they leave damaging yellow creep as they slide.

Yellow Ice Block Gapers are there for usually more dangerous than their non-yellow counterparts.

Snowbob
Snowbobs act as regular Gapers and directly follow the player, doing contact damage. When they die, their head flies off and lands on the floor and can be killed. If it isn't killed in a short amount of time, the Snowbob head will perform actions depending on what kind of head it is.

Sickie
Sickies will follow the player, although they cannot move diagonally, only able to move up, down, left, or right. When they get in range of the player, they will sneeze towards them, sending a cluster of tears that do one full heart of damage. These tears will stick to objects, such as rocks; other enemies; and pickups on the ground, such as keys. While stuck, these tears can still damage the player. Eventually they will unstick and fall off of whatever object they were attached to.

Rag Gaper
Rag Gaper directly follows the player. If the player gets too close, Rag Gaper's head will roll off the body, destroying it, and start rolling on the ground, following the player. The head will continue to decrease in speed and will die if it stops moving.

When buffed, Rag Gaper will become faster and his rolling head will last longer. When his head falls off while buffed, instead of destroying the body, a Rag Gusher will be left. If the Rag Gusher is not destroyed in a short time, another head will regenerate.

Arrowhead
Arrowhead will wander around the room. If Arrowhead is damaged by any means, he will notice the player and directly charge him. After a while of charging the player, Arrowhead will send tears out in a + pattern, an x pattern, and another + pattern, and then return to wandering.

Trivia

 * When an Arrowhead spots the player, he uses the same sound clip that Gemini made in the original Flash game of the Binding of Isaac.
 * In the sin update trailer, an Arrowhead is seen towards the end. He roars as he walks into the frame before inspecting Dante and Charon in their high-five pose. The Arrowhead then either feels awkward or gets bored and paces back offscreen.