fastest horse is the Spirit Steed. I *think* it costs 50p and requires the guild you are in to be lvl 30. Speed is 48% I believe.
Eldarrian Charger is roughly 9p and is 40% speed.
Cant remember the names after that, but you can figure out speed quality by price.
Lowest price is 2.5p for 24% speed?
Mid range is 3.8p for 34% speed?
Or create a Paladin/ShadowKnight and buy 10% speed horse for 57s (at character level 20)