Try to find a mature or aged horse. Old, venerable, and adolescent can also be ridden and hitched to carts, but are not so good for breeding. Old and venerable because they are getting too old (venerable parents have a chance to give their offspring [[animal husbandry|deformities]]. Adolescent horses because they can't breed yet.
Once you find a horse, activate the rope, right click on it, and select Lead. If you BC (body control) is over 21, you can right click on it again and select Ride. Repeat as necessary to use all your ropes (if you are riding, the rope you used to lead your mount is free to lead another animal).
Now you've done it! You've started your own horse farm!