So there was this bloke in the 3rd Century who was a christian scholar (probably big business at the time), named Hippolytus. He spent his time arguing how the Greek Philosophers were full of shit, and also ungodly (which let's face it is probably the main thing that pissed off Hippolytus).
So in the end Hippolytus was matyred (probably by Greeks), by being pulled apart by wild horses, which lead some bright spark to make him the patron saint of horses (they made him a saint, even though at the time he was also the first Antipope).
But then there was this bloke from greek mythology, also named Hippolytus, and he was the "unleasher of horses". (This Hippolytus supposedly also spurned Aphrodite, which suggests he wasn't into chicks because wasn't she like the hottest tart going around?)
So how did this co-incidence with the horses happen? Someone just made up a story of course. Probably the blokes who killed Hippolytus the Saint – they simply stabbed him, but then spread the story that they employed wild horses to do the job because who's going to fuck with blokes who go around pulling people apart with wild horses? Especially ones so learned in Greek mythology.
Man the shit you learn on Wikipedia.