Most '70s-'80s kids know the ultra-annoying Scrappy Doo, who was the Cousin Oliver of the Scooby universe. According to Wikipedia, though, people actually liked him at the time. ("Scrappy-Doo was added to the cast of Scooby-Doo to save the show's ratings, which by 1979 had begun to sink to the point of cancellation threats from ABC. After his addition to the show proved to be a ratings success, Hanna-Barbera restructured the show around Scrappy in 1980. ") What? I was a little old for cartoons when he was brought on, but I've never heard of anyone admiting to even tolerating that bratty little pup. As this Cartoon Network ad admits.
But possibly even worse was the gray dog on the left-hand side, the so-forgettable Scooby-Dum. (Voiced by the amazing Daws Butler, who did Huckleberry Hound, Yogi Bear and so many other cartoon greats.) Again, Wikipedia is hilarious about him. Is he Scooby's brother? His cousin? Both? Is this a southern swamp "I'm my own grandpaw" lineage thing? Best line: " The inclusion of Scooby-Dum was considered one of the first missteps in Scooby-Doo cartoons."
I think their first clue should've been that Scooby-Doo and Scooby-Dum had an incredibly complicated handshake-and-greeting procedure. And also the fact that "Dum" was LITERALLY IN HIS NAME.