actually, horses were originally hunted for their meat, or herded for milk, meat and hides, and only later domesticated for travel.
and now Saudi archeologists are finding clues saying horses were domesticated 3000 years earlier than was believed:
Saudi officials say archaeologists have begun excavating a site that suggests horses were domesticated 9,000 years ago in the Arabian Peninsula.The vice-president of the Saudi Commission for Tourism and Antiquities said the discovery at al-Maqar challenged the theory it first took place 5,500 years ago in Central Asia...
Wikipedia article here.
The first undisputed evidence for their domestication dates back to 2,000 BC, when horses were buried with chariots. By 1,000 BC, domestication had spread through Europe, Asia and North Africa.