S1·E55 Heathcliff Pumps Iron

A big, muscle-bound cat named Pecs tries to take Sonja away from Heathcliff, so he starts working out in order to beat Pecs at his own weightlifting game. Unfortunately, the manager of the gym is in cahoots with Pecs, and throws Heathcliff every possible curve. Heathcliff catches on, and throws the curves right back. Sonja is his in the end.