User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-6050823-20130926215547/@comment-4009941-20130926222423

I have seen no official word of each. But I can say that some appear to be mutations while other just appear older for example HC Gravios all of its changes just hint to older age while HC Black Diablos has 1 crazy long horn, more spikes, Purple flesh and Blue eyes and for Doragyurosu he kind of was a Beru Mutation from the start kinda like Azure Los is from Red Los only more extreme and adapted to cold.

To sum it up I am only going by what they appear but it looks very clear that some are just older but keep in mind looks can be deceiving.

Anyway as for some HC only items info HC Gravios has some Lava like shell on it, HC Black Diablos unusual horns are said to be from very violent black DIablos(I think I got that hint when it started going at double the speed in rage...), HC Doragyurosu's giant claws are said to shock and burn you upon even a slight touch with your finger tips, HC Rathalos scales seem to be burning hot to the touch.