Complex sentence: One of the first horses on the island for which there was a record of importation was the stallion, Jolly, who was taken there in 1801, but there were already horses there and it is thought they may have arrived as early as the 16th century.
Split into simpler sentences: One of the first horses on the island for which there was a record of importation was the stallion, Jolly, who was taken there in 1801. There were already horses living on the island prior to his arrival, it is thought they may have arrived as early as the 16th century.