As I understand it (and I could be wrong) it's as follows.
Those that pre-ordered the imperial edition get a white basic horse, equivalent stats to the in game 17,200 gold one, that's not available anywhere else.
Those that also want a horse straight away but don''t want to spend £20 extra on the imperial edition, can pay something like £10 for a Palomini coloured basic horse ( also same stats as the 17k one)
Those that don't want to spend £10 can wait until they have 17k gold and buy one.
We talk about gold and it seems a lot, but there's no silver or copper in this game. Over the last beta weekend, I had 4000 g at one point with a new character and I was only messing around, I don't think 17k will take more than about a week to get if you really want one. (Compare that to Lotros horse initially, took me months to get).
All of the above horses come with
Speed 15%
Stamina 10
inv 0
You can feed the horse up over many days and this can give you one of the following.
Speed to move quicker
Stamina so that you can sprint on horse longer and take more hits before getting dismounted
Inv which adds slots to your inv all the time you have the horse with you regardless of whether you are riding it or not.
Alternatively, if you have the gold, you can spend 42,700k on one of the following
Draft Horse optimised for carrying
Speed 15%
Stamina 10
Inv 10
Galted Horse Optimised for stamina
Speed 15%
Stamina 20
inv 0
Light Horse optimised for speed
Speed 25%
Stamina 10
Inv 0
I don't know exactly how it works having only played beta for a few weekends, but if the cost remains the same for each feed (250g)
I buy the cheap horse for 17,200 and spend 2500 buying 10 inv levels, it would have cost me a total of 19,500 rather than 42,700 for the expensive horse with the same stats. However I would have to wait 10 days worth of feeding to get there.
I've googled this a lot, and it seems (there seems to be some confusion) that each stat is capped at 50 and the expensive horse gives you a 10 stat bonus, meaning you still get 50 points over about 50 days to spend on your horse, but if you had the inv horse, as it already has 10 in the inv, you could only put another 40 in but that would then leave you with 10 to spend in one of the other 2, but again, it seems a bit vague how it works.
So it's far cheaper to get the 17k horse and feed it up but you will loose out on 10 overall stat points (if the posts I read, the people understood it properly, and they might not have)