When Turbine decides to feature one or two of them in the store. As far as I can see, there is no particular rhyme nor reason to it - when a horse is introduced it will be featured, but short of that, it's - whenever.
Which is frustrating, I know. I love the Steed of Night, won it on one of my characters. Steed of Bright Days I also won on one of my characters, but was thrilled when she won a Lossoth in an envelope so she wouldn't have to ride that horrible thing anymore.
I don't get it, either - lots of folks would pay real money to be able to get a fast horse for their toon with a certain cosmetic appearance, but they don't feature them in the store all that often.
Steed of the Champion is a bit uninspired and has some clipping issues, but it's not bad-looking at all. It's one I do use (won it on a toon) without embarassment, though if you're going to pay 20$ for something, I agree that it ought to be the horse you want.
It may never appear. My favorite is the carmello horse which was briefly in the lottery just before the big lottery crash last year (when it went away for many months). I won 2 of them and would not mind having it on my main character but it remains a pretty rare mount. On the other hand, it is neat to have something that few other people have, and putting them in the store ruins that. A few should always be hard to get.
"stack all the morale you want but dont come on here wonderin why you aint hittin hard! "