Bony

Bonies are skeleton enemies which wander around the room and quickly throw bone projectiles at Isaac when he lines up with them.

Black Bony
Black Bonies are harder versions of Bonies that behave identically to regular Bonies, but fire three bones in a row and explode upon death. The first bone will be fired when Isaac lines up, but the second and third one may be fired at an angle towards him.

Revenant
Revenants are variants of Bonies that appear in the Mausoleum and Gehenna. Revenants wander around the room randomly, occasionally stopping to shoot a stream of flames at Isaac. These flames have limited range, and are blocked by obstacles.

Quad Revenant
Quad Revenants act similarly to normal Revenants, but they shoot a 4-way flamethrower that rotates slowly.

Maze Roamer
Maze Roamers are -exclusive variants of Bonies that shoot eyeballs at Isaac when lined up. They always spawn in pairs, with their movements and attacks being mirrored between each other. When one dies, the other becomes angry and begins directly pursuing Isaac. The opposite Maze Roamer can be determined by its blood stained rags and mirrored eye socket.

Holy Bony
Holy Bonies are larger and tougher versions of Bonies that were introduced in The Binding of Isaac: Repentance. Its bone projectiles travel in an arc and create a blue flame at its landing spot that blocks shots, deals contact damage, and despawns after some time.

Trivia

 * Despite Bonies being made entirely of bones, they leave bloodstains behind upon death.
 * They only leave behind bones when killed.
 * Bonies are one of a few enemies in-game to have their own unique projectile type.
 * Bonies were confirmed to be the first enemy not designed by Edmund McMillen. He also confirmed that it was designed by Matt, the lead artist for The Binding Of Isaac: Rebirth, and that Edmund loved the simplicity of the AI, fitting "perfectly into the isaac world."
 * Bonies are spawned by The Forsaken, and Black Bonies are spawned by its Black Champion version.
 * Holy Bonies, Holy Mulligans, and Holy Eyes originated as cards from the card game The Binding of Isaac: Four Souls.