Chet and Snooks are a duo of vaudeville-like Anything Muppets who randomly pass through Don Music's studio while he and Kermit are trying to figure out the next letter in The Alphabet Song. Snooks asks Chet if he knows why the fireman wears red suspenders and Chet eagerly tells him to say WHY, which causes Don to remember which letter he's looking for. A minute later, they reappear with the rest of the company to join in the song.
These two clowns are a reference to Chet O'Brien who played Mr. Macintosh on Sesame Street and his brother Mortimer, who had been a team on vaudeville in their younger days. They were also referenced in a brief exchange of dialogue in an Old West sketch from season 10.