Mega Ninetales is a dual fairy-fire type with the ability Solar Power, which boosts Sp. Attack but lowers HP in sunshine. The creator did note that the use of Drought as Ninetales' initial, un-evolved ability would create a situation in which the mega evolution is terribly overpowered for five turns, assuming the other team has no counters to weather-based setups.
There are some flaws in the changes in game mechanics it might cause, though if Nintendo were ever to create a Mega Ninetales, this would most likely be avoided. In such a form, Ninetales is even more reminiscent of a kitsune, a Shinto fox spirit. Since it is so fitting, why is this not a thing?