Wendell the Baker for Cinnamon Toast Crunch used to have two other bakers named Bob and Quello/Quienno by some, but they disappeared for reasons unknown around the early '90s, and were never featured again.
McDonaldland and the vast cast of characters who once existed side-by-side with Ronald McDonald in McDonald's commercials were steadily eliminated from the 1970s onward, until only the "core cast" of Ronald, Grimace, Birdie, and Hamburglar remained. For awhile past 2000 now, McDonald's ads have only featured Ronald, and lately even he hasn't been seen terribly often.
The Burger King Kingdom was BK's answer to McDonaldland, and featured a colorful cast of characters including the milkshake-craving knight Sir Shakes-a-Lot, the picture-framed Burger Thing, the robotic Wizard of Fries, the skeptic Duke of Doubt, and The Burger King himself. They were phased out in favor of the BK Kids' Club in the late '80s, which itself disappeared shortly into the new millennium.
Cookie Crisp cereal had in its early marketing campaigns a bobby who chases after a bandit-masked thief and his bandit-masked dog. Eventually the thief vanished, followed promptly by the bobby, leaving just the dog (still wearing his bandit mask, oddly enough). Now the dog has completely vanished, having been replaced by a wolf named Chip (ironically the same name as the dog that preceded him).