Not only did Hanna-Barbera have an advertising deal with Kellogg's, but with rival General Foods, then the parent of Post Cereals, as well.
Post, you see, sponsored Ruff & Reddy, H-B's 1st series for NBC. The cat & dog team (Don Messick & Daws Butler) have to chase away an alien who wants some Alpha-Bits cereal so his world can be in position to take over other worlds down the road, although it isn't implied as much.
Edit, 4/16/18: Had to change the video. Now, we have a block of commercials not just for Alpha-Bits, but for other Post cereals, too.