Actually, the first "Cookie Monster" appearance was before this (http://en.wikipedia.org/wiki/Cookie_Monster):
"In 1966, Henson drew three monsters that ate cookies and appeared in a General Foods commercial that featured three crunchy snack foods: Wheels, Crowns and Flutes. Each snack was represented by a different monster. The Wheel-Stealer was a short, fuzzy monster with wonky eyes and sharply pointed teeth. The Flute-Snatcher was a speed demon with a long, sharp nose and windblown hair. The Crown-Grabber was a hulk of a monster with a Boris Karloff accent and teeth that resembled giant knitting needles.These monsters had big appetites for the snack foods (like cookies) they were named after. Each time the Muppet narrator, a human-looking fellow, fixes himself a tray of Wheels, Flutes and Crowns, they disappear before he can eat them. One by one, the monsters sneak in and zoom away with the snacks.
In 1967, Henson used the "Wheel-Stealer" puppet for an IBM training film called Coffee Break Machine. In the sketch, called "The Computer Dinner", the monster (with frightening eyes and fangs) devours a complex machine as the machine describes its purpose and construction. At the end of the sketch, the talking machine explains that its primary purpose is to produce the greatest explosion known to man. The monster promptly explodes. This sketch was also performed in October, 1967 on The Ed Sullivan Show. It was also later performed on the George Burns episode of The Muppet Show using the Luncheon Counter Monster.
Two years later, Henson pulled the puppet out of the box again for three commercials selling Munchos, a Frito-Lay potato chip. This time, the puppet was called Arnold, the Munching Monster. After the three ads were produced, Henson had the opportunity to renew the contract. He chose not to, because at that point he was working on Sesame Street — and that monster puppet was moving on to the next stage in his career."