Complex sentence: The first Heck horse in the United States, a stallion named Duke, was imported in 1954 by the Chicago Zoological Park in Illinois, followed by two mares in 1955.
Split into simpler sentences: The first Heck horse, a stallion named Duke, was exported to the United States in 1954. He was imported by the Chicago Zoological Park in Illinois, followed by two mares in 1955.