Arelith

Sorcerers, Wizards, Clerics, and Favored Souls (new Warlocks too? I don't know), depending on their alignment, receive a special horse appearance (Nightmare for Evil, White Charger for good) when reaching Cavalier level 6. The idea is cool, but I can't see those classes taking 6 Cavalier levels just for a new horse model, it's not worth the 2 CL sacrifice. In my opinion, it would be more fair if they got it on Cavalier level 4. It's after all a cosmetic upgrade, there's no mechanical advantage there.

Likely to avoid a similar issue to the often occurring reason why plane-touched races and RDD get bumped from greater to major; it's to avoid commonly seeing something rare and exotic far too often.

Those classes are frankly never taking Cavalier, anyways, due to the reason you described.