Frequent flier Running Stag has been retired from racing due to a recurrence of a ligament injury, trainer Philip Mitchell told the Racing Post Monday. The 6-year old's career concludes with seven wins in 40 starts and career earnings of $1,663,227."Two weeks ago the old injury started to rear its ugly head again," Mitchell told the Racing Post. "We just thought we owed him too much to put him under any pressure. He's a once-in-a-lifetime horse and has given us three years of utter delight. The way he has developed has been a fairytale."Running Stag (Cozzene--Fruhlingstag, by Orsini II) won the 2000 Massachusetts Handicap (gr. II) and both the 1999 Brooklyn (gr. II) and Saratoga Breeders' Cup (gr. II) Handicaps. In his five-year career, he raced in six different countries on turf and dirt. His last race was the Arlington Million (gr. IT), in which he finished sixth.