User blog comment:Kogath/Monster Hunter XX Discussion/@comment-17735225-20170324115315/@comment-26421638-20170324120649

I don't have a true answer, but i can say what i know so far...

Most of the deviants, but not all, are (as you said) special individuals who survived to previous hunts (or other monsters) and they gained new traits. MD Diablos is a clear example but also any deviants who got wounds on their body like Deadeye or Heavenly Eye are good examples too. It was stated in a video who introduced Deviants to western hunters,after MHX/MHGen was announced to NA/EU.

Then there are monsters classified as Deviants even if in their lore or ecology doesn't mention at all if they were survivor from previous battles or not, one of this type of Deviants is Crystalbeard.

And then there are also Deviants with unknown origins like Dreadking Rathalos.