Tony Jr., who was on the first Frosted Flakes boxes with his father Tony (who replaced Katy the Kangaroo). He disappeared in 1956, only to come back as a Brick Joke in 1977 as the mascot of Frosted Rice.
When Frosted Rice moved over to be a product of the Rice Krispies line, Tony Jr. again disappeared.
Subverted with the ever-popular "Jack" mascot of Jack in the Box restaurants was originally...well, a jack-in-the-box. He was blown up, box and all, in a memorable 1980 TV commerical that at the time was supposed to symbolize Jack's attempting to rebrand itself as a more "sophisticated" restaurant. Fifteen years later, after the image makeover had failed miserably, Jack returned, "thanks to the miracle of plastic surgery", Jack returned as a human with a big plastic clown's head and took his revenge on the advertisers by blowing them up.
The most recent boxes don't even seem to have Wendell on them.
There was a British TV campaign in which an alien from the planet Zarg  claimed his world was dying and could only be saved by a soft drink called Quosh. Nothing was ever heard from Zarg again, and one can only fear the worst.