South Park (1997)

S1E4 Big Gay Al's Big Gay Boat Ride

Stan has just welcomed a new dog, Sparky, and boasts that he’s the toughest pooch around. However, while Stan believes Sparky is all set for a fight, the dog unexpectedly starts humping Sylvester, leaving everyone in stitches. The other boys mock Stan about his "feminine" dog, prompting Sparky to run off in search of someone who will appreciate him for who he truly is.