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The horse (Equus caballus) is an odd-toed ungulate mammal, one of eight living species of the family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today. Humans began to domesticate horses around 4500 BC, with domestication of the horse believed to have been widespread by 3000 BC. By 2000 BC the use of domesticated horses had spread throughout the Eurasian continent. While most horses today are domesticated, there are still endangered populations of the Przewalski's Horse, the only remaining true wild horse that has no domesticated ancestors, as well as more common feral horses, such as the American Mustang, which live in the wild but are descended from domesticated ancestors.
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Elephant vs Blue Whale, 
