Japanese Racehorse. Sired by Sunday Silence and foaled by Golden Sash (herself sired by Dictus, a very temperamental but successful French racehorse), he was blessed with speed and sheer durability but cursed(?) by the compounded temperament of his parents.
He is best known as the sire of Orfevre (A temperamental racehorse who won the 2011 Japanese Triple Crown), Gold Ship (An unpredictable racehorse who was almost as successful as Orfevre), and Oju Chosan (an astounding 6 time winner of the Nakayama Grand Jump, as well as the 3 time winner of the Nakayama Daishogai).
In spite of the success of his offspring, the two above who ended up as the luminaries of their era, Stay Gold himself only won a single GI race in his 50 race-long career (his final race at the 2001 Hong Kong Vase) and tended to collect second places. To be fair, some of this has to do with the fact that he had to contend with the luminaries of his era (Special Week and T.M. Opera O among others).
In the context of Umamusume, aside from Gold Ship being a popular character and Orfevre having at least an early design, he is also the real-life sire of Nakayama Festa. Otherwise, a stand-in of him appears in the first season of the anime (see Kin'iro Ryotei).