Complex sentence: Kisber Stud was very successful in this; their best known horse was a mare called Kincsem, who won 54 races and was entered in The Guinness Book of World Records for it.
Split into simpler sentences: Kisber Stud was very successful in this, and was reflected most notably in a mare called Kincsem. Kincsem won an amazing 54 races and was entered in The Guinness Book of World Records for it.