Argent Tournament

The Argent Tournament has a variety of different mounts to earn and can be a fairly long grind. To obtain them you will need to complete quests for the Argent Tournament which is located North East in Icecrown. You will eventually begin to earn Champion's Seals from completing dailies which is what you will need to purchase the mounts. Some will require you to be exalted with a specific faction (race factions) or Sunreavers (horde) depending on which mount it is.

Here are two that I have collected:

Sunreaver Dragonhawk
Requires Sunreavers - Exalted
Costs 100 Champion's Seals

Argent Hippogryph
Costs 150 Champion's Seals