Horses being able to sprint is a vanilla feature and it has been around for quite a while.
Really?! Played for a year or two now and never noticed - to be fair I did only relatively-recently discover that there is a key for sprinting other than double-pressing the forward key (which I don't think does work on horses but might be wrong?). Are horses sprinting with the sprint key faster than when they run without sprinting? Or is it just a FOV effect as Fallen_Snipez mentioned? Please and thanks!