I’ve done a quick search, here and on google, and I can’t find out exactally how and when this phrase originated. I have a pretty good idea it was back when horses were the main way of transportation. I would like to know when the phrase started, and why we still use it , even though I’m sure it makes no sense to most of the population.
My slang dictionary dates it to the 1840s (first-known use, 1844). I’d say it’s still used because it’s so vivid: someone holding back a pair of rearing horses.