Unless it's literally just handed to you when you reach max level/Valdrakken then it obviously has to come from somewhere, and given it's appearance it seems a safe bet it would be considered a somewhat significant reward.
The obvious candidates for this are endgame questlines, reputations/Covenants 2.0, and open world exploration, of which Dragonriding is supposed to be a large part.
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It absolutely could just be a customization option as a reward for all Gold races, but that mount has to come from somewhere, and unless that somewhere is when you reach max level it seems to me like it would be a reward of some kind.