User blog comment:Aandrew07/Discussion of the Week: Monster Variants/@comment-87.65.141.234-20121201161051/@comment-4212769-20121201164347

They're called subspecies in the guild because you get different materials from. Black Diablos for example is a Diablos in Mating Season. If there wasn't such distinction you would go for a Diablos for its materials but end up with a Black Diablos.

But as all monster here (but Hungry Deviljho) don't give different materials, they cannot be considered as subspecies anyway.

Technically you're right but in-game they are subspecies.

Also Ashen Lao-Shan Lung isn't older than the red one. It's just the different area he wandered in.