To add to the topic:
-Mounts can be captured. For example, human swordsman on foot, by some miracle managed to kill Elven ranged on a unicorn. Unicorn is now his, if there are no other elfs around.
-Mounts can be shared between races (mounts act very much like sheep in Age of Empires III – it`s possible to capture them.) Some races will have better/cheaper mounts, so on some level it would make sense for Orcish player to train alot of wolf-beasts (the toughest mount in-game) and send them to his human allies to mount some of his elite Knights. Creating the toughest mounting cavalry in-game.