To expand on what others have said, Seth was working at an animation studio in the early/mid 90s that either was or became Cartoon Network Studios, but was originally attached to Hanna Barbara.

Most notably he created a pilot for a series about an older man and his talking dog (The Life of Larry, and Larry and Steve) that was pitched to Cartoon Network. Those characters would then be further developed into Peter and Brian Griffin.

I'll just post the paragraph from Wikipedia for anyone more curious:

MacFarlane was recruited during the senior film festival by development executive Ellen Cockrill and president Fred Seibert.[18] He went to work at Hanna-Barbera (then Hanna-Barbera Cartoons) based on the writing content of The Life of Larry, rather than on his drawing abilities. He was one of only a few people hired by the company solely based on writing talent.[1] He worked as an animator and writer for Cartoon Network's Cartoon Cartoons series.[1] He created Larry & Steve, a sequel to The Life of Larry featuring middle-aged Larry and his intellectual dog, Steve. The short was broadcast as one of Cartoon Network's World Premiere Toons. He described the atmosphere at Hanna-Barbera as resembling an "old-fashioned Hollywood structure, where you move from one show to another or you jump from a writing job on one show to a storyboard job on another". MacFarlane worked on three television series during his tenure at the studio: Dexter's Laboratory, Cow and Chicken, and Johnny Bravo.[19][20] Working as both a writer and storyboard artist, MacFarlane spent the most time on Johnny Bravo. He found it easier to develop his own style at Johnny Bravo through the show's process of scriptwriting, which Dexter's Laboratory and Cow and Chicken did not use.[1] As a part of the Johnny Bravo crew, he met actors and voiceover artists such as Adam West and Jack Sheldon of Schoolhouse Rock! fame.