Sucker

The Sucker is a fly with a large abdomen, and has a greater amount of health than a regular Fly and slowly moves towards Isaac. Upon death, it shoots 4 bullets in all cardinal directions. Suckers are unaffected by, so they're always hostile.

Despite their simplistic behavior, Suckers can prove to be rather dangerous at late levels. This is because they're often positioned in areas containing hazards, or the player may have familiars that attack the Suckers regardless of the player's intention. As a result, the low health that the Suckers possess can be used to give the player little time to react when they die quickly and shoot their projectiles outwards.

Spit
The Spit looks like a green version of the Sucker, and it shoots one explosive green bullet at the player upon death. The shot behaves exactly like. Spits are unaffected by, so they're always hostile. Like most enemies that possess explosive attacks, Spits can be used to bomb tinted rocks or open Secret Rooms.

Soul Sucker
Soul Suckers were added in The Binding of Isaac: Afterbirth †.

The Soul Sucker looks like a bloated, red-spotted version of the Sucker. Upon death, a swirling red vortex appears where it died, which then fires four beams in the cardinal directions after a short delay.

The appearance of the vortex resembles the same vortex that is spawned whenever the player fires with and.

Ink
Inks were added in The Binding of Isaac: Repentance.

The Ink is simply a tar variant of a Sucker. Upon death, instead of shooting four blood shots in the cardinal directions, it shoots four tar shots in the ordinal directions as well as a puddle of slowing black creep.

Mama Fly
Mama Flies are a new addition to The Binding of Isaac: Repentance.

They fly around the room away from Isaac and plant down Pustules. They spawn Small Maggots upon death.

Tainted Sucker
Tainted Suckers are larger and more corrupt versions of Suckers. Upon death, they shoot 6 large blood shots that become 6 smaller shots upon contact with a wall or obstacle.

They are exclusive to the Ascent to.

Trivia

 * Suckers seem to be based on mosquitoes, midge-like flies that suck the blood of other animals, as they seem to share a small proboscis and blood sac.
 * Soul Suckers were added in Booster Pack, after the initial release of the DLC Afterbirth †.
 * Tainted Suckers have the lowest health out of any tainted variant, second is Tainted Boom Fly.