There are a number of posts depicting Princess Celestia with a fiery mane and tail. The MLP wikia names her as Nightmare Star (here and here ).

Here on E621, she is mostly referred to as solar_flare_(mlp) (16).
However, there are also tags for nightmare_star_(mlp) (9), nightmare_celestia (1), solar_flare (5), and solar_flare_(character) (1).
Most of these can be found by searching princess_celestia_(mlp) fire (191).

At the time of writing, none of these tags imply Princess Celestia or My Little Pony.

• Should one of these be used as the primary tag? A different tag, not yet mentioned?
• Should these tags exist at all? Since most these posts can be found with a simple search.

For comparison, derpibooru aliases both nightmare celestia and solar flare to nightmare star.

Considering the significant and consistent differences between the original character and this other form, I'd say it's deserving of it's own main tag. Whichever gets chosen the others should get aliased to, excepting solar_flare, which I think is too general—there's already overlap—and instead should be disambiguated.

SaimonPSmith said:
For comparison, derpibooru aliases both nightmare celestia and solar flare to nightmare star.

Might as well follow precedent and make nightmare_star_(mlp) the standard, no? I'm uninformed on MLP though, so I could be wrong.