As Scotty would say, You cannot change the laws of physics!
If you have a 10 second clip of a horse running, I don't know any way to make it a 15 second clip without slowing it down.
Although I suppose you can insert some clips within it. Reaction shots of an audience, for instance, or close-ups of the person riding the horse. Maybe even a cutaway shot of a stopwatch or clock to show time passing.