Modern horses are believed to be descended from four primitive horse types; a pony type thought to exist in northwest Europe, a pony thought to exist in northern Europe and Asia, a horse thought to exist in Central Asia, and a desert horse thought to have lived in Western Asia. Fossil evidence suggests that these were the ancestors of all pony and horse breeds.  Until the arrival of Spanish explorers horses were extinct throughout the Americas. There is fossil evidence of prehistoric horses on the western continents. Changing climate may have forced primitive horses across a land bridge to northern Asia. Any wild horses now in existence such as the Mustang, or Ponies of Assateague Island are feral—having escaped from captivity and adapted to their new environment.  Relatives Mules, a cross between a male donkey (jack) and a female horse (mare), are hybrid and generally can not reproduce. Hinny’s are the offspring of a female donkey (jenny or jennet) and a male horse (stallion).  Preswalski’s horses are thought to be the last true wild horse. Other members of the Equus family are onagers, zebras, asses, and kiangs. Each of these has evolved to live in their particular environment—often hot arid conditions that would not support a horse.

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