For a tag to count for character categorization, it must be featured in any official MLP media (animation, toys, merchandise, comics, etc) of any generation, pony or not, and not be a variation on an existing character. (like Princess Twilight or Dusk Shine, those are still Twilight Sparkle so only the main name is colored)
Here’s a good list of all the G3 pony releases by name, so all the names can be searched here and ones not marked as character can be.
This is the closest on the same site for G1 ponies, by country, tho US and UK are really the only important ones.
For G2, uhh this is the closest on the same site, but, it’s inefficient, and probably missing some.
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