K'nuckles II (pronounced K'nuckles the Second) is Flapjack's pet rat in the episode K'nuckles is a Filthy Rat. Although a generally friendly creature, he had the power to throw fleas upon anybody who was mean to Flapjack (and/or himself). Anybody who was hit by a flea were instantly infected by the Bubonic Plague. Although he only appeared once, he played a major role in the episode. At the end he was sent to Plague Island where he met a female rat and fell in love with her leaving Flapjack without saying goodbye.
K'nuckles the ll is dark brown and his eyes are black, but they turn red when he gets angry and when he growls. He also wears a red collar around his neck.