Whipper

Whippers are fiendish enemies added in The Binding of Isaac: Repentance.

They wield long fleshy whip-like tendrils, which they use for a short-range melee attack. They wander around the room in straight lines and charge at Isaac if he lines up with them. They open their mouths when charging to show that they are attempting to harm the player. If the player gets too close, they will slash their whips at him, damaging Isaac if he is too close and deflecting nearby tears. Whippers will also whip at placed bombs.

In Gehenna, their appearance changes to resemble a gimp.

Snapper
Snappers are much more dangerous versions of the Whipper. They possess a much longer, fleshy whip that can extend across the room, at the minor cost of decreased attack width. They behave similarly to normal Whippers, but will snap their whips at Isaac if he lines up with them instead of charging. If Isaac is far enough away, they will still charge at Isaac before snapping their whips. Similar to the Whippers, their appearance resembles a gimp in Gehenna.

Flagellant
Flagellants are even more mutilated versions of the Whipper. They wander around the room in straight lines, charging at Isaac relentlessly if he lines up with them. However, they constantly swipe their whips back and forth as they charge, damaging Isaac if he gets caught and deflecting tears much more frequently than regular Whippers. Flagellants are also more aggressive than regular Whippers and will chase Isaac for a while instead of simply charging once.

Trivia

 * Flagellants were formerly called "Lunatics" in the Antibirth mod.
 * The names of the Whipper and Snapper enemies are a joke to the term "whippersnapper", which is the name for a young and inexperienced person who is considered to be presumptuous or overconfident.
 * Whipper, Snapper, and Flagellant's sprites were changed slightly in Repentance.