It takes a "Knight" + a "WarHorse" = "Mounted Calvary"
The horse by itself is nothing more than a highly trained, to not freak out under fire basically, unguided animal.
The Knight, without his Mount, is nothing more than an un-weildy, slow, but heavily armored Chess Piece, on the Battlefeild. He isn't running around much at all and certainly isn't running anywhere's in any kinda of hurry.
But, when you combine the two, you get the very serious battle ready "Mounted Calvary" unit. And in number wreak absolute havoc to the enemy foot soldiers they find.
Sadly, if at any point during the battle, the pair once again become re-seperated, you are back to the same ole solo units.
The knight may be able to defend himself, but he won't be getting back on to his mount alone, nor will the mount begin to start stomping on those aound him in the frey. How does he know friend from foe, other than his own rider when the stuff hits the fan.
Each compliments the other to such an extent that when alone, they are basically useless, per say, under the type of conditions in which we a likely to find both in the game proper.