I'm curious as to why the latter tag is so undertagged. In fact, only these three images are tagged with mythical_pokémon. The wiki page for mythical_pokémon is only 2 years old, and while that's not that old in the grand scheme of things, considering the age of this website, it's still plenty of time to use the tag more.

In addition, on the Legendary Pokémon page, these Pokémon are mistakenly listed as Legendary, and not referred to as Mythical, as they should be:

So, for clarification purposes, it would involve merely shifting all these, plus the Meltan line and Zeraora, to the mythical_pokémon wiki page to clear up and correct the wiki pages themselves. Not sure if I should include the grouping terms like mew_duo without the additional non-Mythicals, but it probably seems like a good idea, since some groupings, like the aforementioned Mew Duo and the Lunar Duo, contain both Legendaries (Mewtwo, Cresselia) and Mythicals (Mew, Darkrai).

Conversely, on the tagging side, 20366 images are tagged as legendary_pokémon, so I do understand that it would be a lot of work to go through them all and fix the ones that specifically feature only Mythical Pokémon and no Legendaries. I suppose if I temporarily blacklisted every (official) Legendary Pokémon by name, that fixing the images would be easier, as I could just search legendary_pokémon and fix the images where appropriate. However, there's the issue of group images. Specifically those containing both categories, which is very likely. There is also the problem that the aforementioned mislabeled Mythical Pokémon, plus Zeraora and Meltan/Melmetal, are implicated to legendary_pokémon, so that might be an utter mess to sort out. I suppose it would involve changing the implications of them all to mythical_pokémon apart from Phione, whose disputed status is a can of worms I don't want to get into.

EDIT: I'm also noticing the individual Pokémon's pages, plus those of the same-gen legendaries, are formatted to include Mythicals under the Legendary listing too, which means even more work.

Wiki written two years ago by someone with a tagging-abuse permaban, and only three posts in the tag.
I'd guess the status quo has already been to retag them to legendary.

Probably should invalidate or alias to legendary_pokémon.

There's no point in having them separate. Alias to legendary_pokemon.

If we alias it out, there should at least be either a mention next to each Mythical's name that it's Mythical, or it should be a separate list on the same page.

AgentParadox said:
If we alias it out, there should at least be either a mention next to each Mythical's name that it's Mythical, or it should be a separate list on the same page.

Go for it.

I have separated them into a new section on the page, including the Mythical Pokémon desc. All that's left is to alias the mythical_pokémon tag to legendary_pokémon.

Edit: I also found a post using the tag mythical_pokemon (without the accent on the é), so that should be aliased away, too.

AgentParadox said:
If we alias it out, there should at least be either a mention next to each Mythical's name that it's Mythical, or it should be a separate list on the same page.

I'd say in the wikis. On the wiki page for Legendary Pokemon, and on the page of any Mythical. No need to throw it onto the tags themselves, though.