Mamoswine's dex entries say that it dates back to 10,000 years in the past, which made me wonder if the Mamoswine of the past were different genetically. And so, Mega Mamoswine is what would happen if those ancient genes were revitalized.
Now, you may think it looks more like a Paradox Form than a Mega Evolution, but that's by design. I used Roaring Moon's resemblance to Mega Salamence as an inspiration.
by AsterMachina