Bloodlines is an enormously powerful upgrade for those civilizations lucky enough to have it. Considering its negligible cost, researching it is a top priority for every player who uses at least one type of mounted unit.
Note that, despite lacking Bloodlines, the Franks still have the most powerful Paladin in the game.
"Warriors in the Middle Ages required horses of different types for long movements, scouting, draft animals, and carrying armored knights. Careful breeding of horses allowed the gradual evolution of different horse bloodlines to meet these needs. Civilizations with access to large grasslands were especially adept at breeding horses. In the Middle East and Asia, similar attention was paid to breeding camels for racing speed or for stamina to withstand lengthy marches as part of trade caravans."