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!
I haven't done any testing but reviewing the wiki pages for both transportation and horses, neither lists sprinting as a horse ability so I think it is just a FOV change. But I still use it because it does give the appearance of moving faster, even if it doesn't actually work. Placebo effect is good enough for me lol.
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A look at the bug tracker appears to confirm that this is only a visual effect and there is no actual speed boost; the sprint key appears to activate the FOV change regardless of whether a player is on the ground (double-tapping W only works in the latter case, at least it in 1.6.4, which does not have a sprint key, although you can get the same effect (until the 30 second sprint-timer wears off) if you sprint before getting in a minecart or boat):
Well, it's not merely a visual effect in my experience because I've noticed a difference in distance happening when I was using horses to get to a remote base of a friend and I was following him. Regular running made me fall behind but sprinting helped me to keep up and get closer.
Still, obviously I can't know for sure what my friend exactly did and if they kept sprinting or slowed down.
One way to settle this though, I'll see if I can come up with a nice test track.