Quisp,

Quisp and Quake were two sugar-sweetened breakfast cereals originally released in 1966 in the U.S. by The Quaker Oats Company and generally advertised together (during the same commercial) as products competing against each other.
The very successful ads were cartoons created by Jay Ward of Rocky and Bullwinkle fame, and used some of the same voices, including Daws Butler as the voice of Quisp (an alien) and William Conrad as the voice of Quake (a miner).

In 1970, a contest was held by Quaker Oats to see whose cereal was the more popular of the two. In 1972, Quisp won, and Quake retreated underground.

Quake, the cartoon character, reappeared in a new cereal called Quangaroos where he obtained an Cary Grant-esque orange kangaroo as well as putting his mining helmet down in favor of an Aussie motif.
In the late-1970s, Quisp was removed from grocery stores due to low sales. It re-emerged in the mid-1980s and was quickly pulled due again to low sales.

They Always Come Back:
Quisp was re-introduced in the 1990s as the "first Internet cereal" and is still in limited distribution. You can buy it direct from their website here.