Horses are not motorcycles.
If your protagonist’s interactions with his/her gallant steed could equally well (with a change of costume) be interactions with his/her Harley-Davidson, then you have a problem.
If you don’t feel comfortable writing the horse stuff, but are dealing with a fictional milieu where horsepower is what you’ve got, then either do the research (as I’ve said here before, horse people are, taken as a group, glad to be helpful in this regard) or keep your characters indoors and on foot as much as possible.
While you’re at it, take a moment to consider whether or not the horses-as-motorcycles issue might be symptomatic of a larger problem with your story. Pre-industrial societies are different from modern ones, even if they’re entirely imaginary, and it takes doing the research (again) to get them right.
Dr. Doyle's Blog 
“Horses are not motorcycles.”
That might be the quote of the week. Great post! You’re absolutely right, horses are not motorcycles, and if you’re going to write about horses or anything in the medieval fantasy realm, know your stuff! A writer who does this exceptionally well is George R.R. Martin. I’m not sure if you’ve read any of his Ice and Fire series, but he knows the technology of the time to the nth degree, and he makes all of it very accessible to us ever day reader laymen.
Great post, and thanks for sharing 😉
I’ve got another rant about the use of fire in fantasy novels, but I’m saving that one for the dead of winter, when it’s forty below outside and I need something to warm my bones.