Someone who became a legend because of what they rode or commanded, armed with his mount, long range movement, high speed and maneuverability they aim to kill the opposition without getting hit.
Example Heroes: Alexander the Great, Perseus
To force the servant you are summoning into the class of Rider you just add these three lines to the end of your summoning chant.
"Through strife thy rode upon the backs of beasts.
Thou, seated upon the throne of movement. Master of mighty mounts, I am he who holds thy reigns."
As listed on the Servant category page.
Their free Class skills are:
Riding Rank D
Riders are then allowed to pick up to six skills for up to 37 points.