It wasn’t the first animated series in primetime but The Flintstones was the first to last more than two seasons. It was also the longest-running one, until it was surpassed by The Simpsons in 1995/96.
Debuting in 1960, The Flintstones ran for six seasons on ABC and was originally geared towards adults. Over time, the show’s humor and storylines became aimed more at a younger audience. By the sixth season, it was clear that the show was running out of steam and, as often happens, the producers added a new character to try to breathe new life into the TV series (the show was still cancelled after April 1966).
On The Brady Bunch, it was Cousin Oliver. In The Flintstones, it was The Great Gazoo (Harvey Korman), a short alien from the future who’s been exiled to Earth. Possessing magic-like powers, he tries to help Fred and Barney and this often leads to lots of trouble.