Argent Knights can be summoned with an for 30 seconds. When this time is up they will bubblehearth, which is not dispellable by any means. Your Faction determines what race it is, humans for the Alliance and blood elves for the Horde. They also use the paladin mounts, again with the colours of their respective faction. As of patch Patch 3.2.0, blood elves spawn for both factions.