I'm confused now. Does it appear commonly, or as rare as finding a strange plant or nymph?
Spawned by the player via relatively common resources, available pre-hardmode.
The reason is that this mob is already so insanely hard to defeat anyway, that only the proest tier players will be able to. And they can choose to do so at any level, but it will be a little bit more difficult the earlier tier you are, given the fact that you lack accessories to aid in your survival or even dodge this beast.