Fly

Flies are very common enemies, found on nearly every floor, including the new floors added in The Binding of Isaac: Antibirth. The item Skatole will change the behavior of most flies.

Willo
Willoes are a type of fly that orbits Isaac. As they orbit Isaac, they will spark up a bit and then fire an orange shot inwards at Isaac.

They can be spawned by Mullighouls.

Dragonfly
Dragonflies are a type of fly that bounce diagonally across a room like Boom Flies. However, when killed, instead of exploding, it will release a stream of fire in either the cardinal or diagonal directions, depending on whether their eyes are opened or closed respectively.

Fly Bomb
Fly Bombs fly erratically towards Isaac and are fairly fast, but do no contact damage. When killed, they will turn into a miniature bomb that explodes after a short delay. This miniature bomb behaves like a regular bomb, damaging Isaac and monsters. These can be spawned by Blasters.
 * Fly Bombs can be instantly killed by using Remote Detonator. This causes them to explode immediately.

Ink
Inks are a type of Sucker that slowly flies towards Isaac. Upon death, it will release 4 black ink shots in the diagonal directions and leave a puddle of slowing black creep.

Mama Fly
Mama Flies slowly fly around aimlessly and periodically spawn Pustules on the ground. Upon death, they release Small Maggots.

Sick Boom Fly
Sick Boom Flies bounce diagonally around a room like Boom Flies. When killed, they spawn a large cloud of poison that will hurt Isaac if he stays in it for too long.