Skinny

A Skinny is a variant of the Gaper with melted faces and a very small Red Creep trail behind them. Skinnies travel much slower than normal Gapers, making them easy to kill without receiving damage.

Rotty
Rotties are paler versions of Skinnies that can turn into Bonys or several Attack Flies after being killed.

Crispy
Crispies are burned Skinny variants. They initially start slow but will turn darker in color until it becomes black and begin to chase the player at a much faster pace as they take damage. They also take damage regardless of Isaac every few moments.

Trivia

 * Unlike Skinnies, Rotties make Gaper noises when chasing the player.
 * Crispies were first revealed on Twitter by Tyrone Rodriguez.
 * The Crispy has been added in Booster Pack, after the initial release of the DLC.