They're not transportation, they're self-propelled breweries:

Horses were domesticated much earlier than previously thought, according to a team of researchers.
...
Writing in Science, a team from the UK's Exeter University suggested that the community in Kazakhstan rode their horses 1,000 years earlier.

They also ate them and drank their milk, possibly as an alcoholic brew.

I've heard enough rumors about fermented horse milk to continue to hope all I ever know about the stuff is rumors.