A steed is horse used for riding. In adventure stories set in the Middle Ages, knights are always galloping around on their trusty steeds.
The noun steed is a very old-fashioned way to say "horse." In Middle English, a steed was distinguished as "a great horse," as opposed to a palfrey, an ordinary, everyday horse. So a steed was typically a war horse that carried warriors into battle. Since the 16th century, this word has primarily been used in literary contexts.