The mountain range is treacherous terrain.
While there's no magical realm that isn't treacherous, it was treacherous in a somewhat different sense from other magical realms.
It wasn't treacherous because it was an extreme environment where vegetation itself could barely survive, like a desert or glacier.
It was because of its physical form.
Sharp rocky mountains were scattered about, and knife-like stone pillars jutted out everywhere.
The highest peaks even had eternal snow that never melts.
Even for powerful monsters, it's nearly impossible to freely roam the mountain range.
Since the terrain is challenging even for monsters.
It's natural that flying-type monsters who are relatively free from these constraints would dominate.
Not as rare as dragons.
Wyverns - monsters much tougher than birds.
As time passed, wyverns essentially became the rulers of the mountain range.
