I suppose they could have included a "horse barbarian" prestige class, but personally I think that WotC are already losing sight of what a prestige class was meant to be! Quite a few don't seem to be "prestigious" and don't have a clear link into a campaign setting or organisation. I prefer prestige classes that exemplify a particular organisation or specialism, and in particular those that give some campaign flavour.
The big question is - what would you expect or like to see in a prestige class aimed at barbarians?
Perhaps something like "Chieftan", a class taken by leaders of the tribes which sacrifice some raging ability in order to gain more leadership - type skills and morale-inspiring abilities?
Perhaps something like "Witch-Doctor" (although that is probably better achieved by multiclassing with Druid or Adept)
Perhaps something like "Speaker With Strangers" - a Barbarian diplomat with social skills and who is more used to handling negotiations with the strange outlanders?
(for one semi-official barbarian prestige class you could find "Rage Mage" in a back issue of the Dragon magazine - they can rage in order to improve their magic spells.)
What do you think?