Complex sentence: Jimmy Croll conditioned horses for Rachel Carpenter for 37 years and who, at the time of her death in August of 1993, owned Croll's most famous horse, the then unraced Holy Bull.
Split into simpler sentences: Jimmy Croll conditioned horses for Rachel Carpenter for 37 years. At the time of her death in August of 1993, she owned Croll's most famous horse, the then unraced Holy Bull.