An F1 (2,3,4,5,6) may be the only one of it's kind, never to be seen again.
But as for rare in nature, South American (and islands near) peppers, not cultivated by natives are the rarest.
Remember, they weren't anywhere else in the world before Columbus brought some back to Spain.
There always exists the possibility of a new one being found in the rain forest, or some variety only cultivated in one village on Bali.
The problem with rare varieties is that chile-heads don't let them be rare for long if they are worthy.
But Bootsieb is correct, and I am sure at least 1 new rare variety will be revealed every year, though it takes at least 4 years (2 seasons each) of hard work to come up with a commercially stable new pepper.