Complex sentence: Images of domesticated horses with leopard spotting patterns appeared in artwork from Ancient Greece through the Early modern period; the Nez Perce people of what today is the United States Pacific Northwest developed the original American breed.
Split into simpler sentences: Images of domesticated horses with leopard spotting patterns appeared in artwork from Ancient Greece through the Early modern period. The Nez Perce people of what today is the United States Pacific Northwest developed the original American breed.