Complex sentence: Southern Image was best known for his 2004 wins in the grade one Pimlico Special and the grade one Santa Anita Handicap, which made him the third ever to win both prestigious races on separate coasts of the United States.
Split into simpler sentences: Southern Image was best known for his wins in the grade one Pimlico Special and the grade one Santa Anita Handicap. In 2004 he became only the third horse ever to have won both prestigious races on separate coasts of the United States.