Unicorn has always been the perfect name. In the beginning it conjures pictures of a majestic, beautiful, and rare animal. And then later you realize it's just a pretend fantasy.

Or a dead horse with unusually twinkly PR.

I like "black swan"

Black swans are very particular and mate for life. I think another metaphor is needed here.

Icaruses? High flying, great pedigree, abundant hubris till they go over the edge unprepared.

Doesn't exactly roll off the tongue. My vote is for "wisps". So fragile, the slightest disturbance in its reality distortion field and it ceases to exist.

Icarhorses

That term already has a well known meaning tho'