The cheapest TCG mount is the strider. Yes, another strider. 25 mounts might be difficult to test, but 10 mounts should be easy enough. Does the felsteed count if you have it and 9 other mounts? The armory in the signature shows 24 mounts. Did you learn the DMF strider? Did you log out, or just quit? Sometimes armory doesn't update if you just quit. Anyway, if I saw you had 25 mounts and no achievement in the armory, that's confirmation, but that's not what we have here.
Edit: Armory has updated now, and it does show 25 mounts and no 25 mounts achievement. Odd. Horde only issue? Lock only? Horde lock only? Hard to say. Not many people have 25 mounts yet.