Complex sentence: He originally used the pseudonym, "Mr Valentine" when he first started to race horses; his horses first trained with John Scott at Malton, North Yorkshire and when Scott died in 1871 he trained with Mathew Dawson at Newmarket.
Split into simpler sentences: He originally used the Pseudonym, "Mr Valentine" when he first started to race horses. His horses first trained with John Scott at Malton, North Yorkshire and when Scott died in 1871 he trained with Mathew Dawson at Newmarket.