I think you should remove the mount you get for obtaining 10 mounts and add a different reward. The reason being is that a good deal (not everyone) of players will collect vanity/achievements.. etc.. to show off. Having a mount that is achieved from learning a number of other mounts can devalue the "worth" that a player may feel when equipping their previous mounts.
My suggestion would be to change this reward to add a visual effect to existing mounts rather than giving a mount. More visual effects could be unlocked with more achievements that may be released (an improvement to an existing effect or effects that can be used in tandem with the others).
These effects could be things such as a "divine glow" allowing a mount to slightly glow with a yellowish light, any more can be thought up quite easily.
I understand this suggestion might hold not much value but it's just something I could make a valuable comment on regarding my experience in high-tier trading throughout a handful of games.
