Complex sentence: These ponies and the annual Pony Penning Day are the subject of Marguerite Henry's 1947 children's book "Misty of Chincoteague", which was made into the 1961 family film "Misty", which was filmed on location.
Split into simpler sentences: These ponies and the annual Pony Penning Day are the subject of Marguerite Henry's 1947 children's book "Misty of Chincoteague". This was made into the 1961 family film "Misty", which was filmed on location.