Once In A Blue Moon

Some go as far as capitalizing the word, but personally I'm against what you presented. Unless we go as far as presuming that Pegasus was actually a thing in Equestria -- and that would have to be pre-Equestria, probably even before the dawn of pony race at large -- the word in and of itself has no other explicit meaning than to denote the winged ponies. Following the trend, you should probably start using 'unicorn ponies' as well, since otherwise that would be the only race left...

But here, in 'our world' it does have that meaning, which is why I use it. Unicorn has no such history and, being a true noun rather than a noun derived from a name, has a more elegant plural form. In any event I do not claim my usage to be canonical, only more elegant - although if I am ever writing a character's dialogue then I should remember to use the canonical plural.

I still don't agree, especially with the elegance bit, but I do respect your point of view. It's also worth noting that MLP constantly transforms common nouns into capitalized names, so I think it's allowed to make one case of doing the opposite as well