"装以马鞍" can be translated as "to saddle (a horse)" or "equip (a horse) with a saddle" in English.
If you're referring to the action of putting a saddle on a horse, you might say:
"Saddle the horse."
"Equip the horse with a saddle."
If you're describing a horse that has already been saddled, you could say:
"The horse is saddled."
"The horse is equipped with a saddle."