Knight's Steed

Vigilance
Mount Knight - {2} (Unless this creature already
has a creature mounting it, you may cast a Knight for
its mana cost reduced by {2}. When you do, attach this
to the creature you cast as it enters the battlefield. The
creatures block and attack together, counting as one
creature. You choose how to assign all combat damage
dealt to them. The mounted creature gains haste and
all keyword abilities of the mount. If it has mount, it
can't be mounted.)