Woody Woodpecker

Over at Something Old, Nothing New, Jaime J. Weinman posts a link to a list featuring descriptions of all the cartoons on the upcoming Woody Woodpecker DVD, and discusses Woody's origin in an Andy Panda cartoon:

He's hardly the only popular cartoon character to start out in someone else's cartoon. Popeye's debut cartoon, "Popeye the Sailor," is sort of an early version of a stealth spinoff pilot -- since it launched the character in a Betty Boop cartoon. Daffy Duck, of course, made his debut in a Porky Pig cartoon.

His readers go on to discuss other characters who started out in another character's cartoon:

Goofy first appeared in a Mickey Mouse cartoon (Mickey's Revue, 1932), and depending on your view on the matter, Bugs Bunny might've made his debut in a Porky cartoon (Porky's Hare Hunt. 1938 d. Ben HArdaway).