Les is a greyhound racer who was the first owner of Santa's Little Helper.
After Santa's Little Helper lost on a Christmas Eve race at Springfield Downs Dog track, Les abandoned him in the parking lot and Homer and Bart took him in.
When Santa's Little Helper became the new mascot for Duff Beer, Les came back and claimed him again to get the money that the dog was getting. Eventually, when a new face for Duff Beer was found, he let the Simpsons have Santa's Little Helper again due to him no longer being worth anything.
 Behind the Laughter
His name is not mentioned in the series, but the book The Simpsons: One Step Beyond Forever gives his name as "Les".