I've written about short-lived cereals before (Urkel-Os!), but The Metal Misfit has a ton of archived posts about flash-in-the-pan breakfast cereals. He calls it Cereals from Beyond.
Totally worth scrolling through, and I've pulled out a few treasures:
--Dinky Donuts: TOTALLY illustrated by that guy who drew Bummers in Dynamite Magazine.
--Croonchy Stars: Whaaaa? The Swedish Chef had a cereal?
--Kaboom: With the rainbow clown. Kinda the generic sugary cereal of sugary cereals. The clown had no cartoon or other tie-in that I could see, so naturally he wasn't quite going to rise as high as Mr T cereal or C-3POs.
--Morning Funnies cereal: Apparently the point was that they put comic strips on the box. Big whoop.
--Cracker Jack had a cereal?
--Vanilly Crunch might be the only one of these cereals that I would regularly buy if it still existed.
--Circus Fun=Kaboom rip-off.
--Fruit Islands=Fruity Pebbles rip-off.
--Waffelos. Man, I can still remember how artificially maple these tasted.
--Breakfast with Barbie. Holy. Crap.
Fruity Yummy Mummy (and Fruit Brute) were the two monster cereals that never made it. And the mummy is so obnoxious in this ad you can see why. He should have been distinguished and with a clipped Egyptian accent, not sound like a small-time mobster from New Jersey.