>>42310281Something that looks like it could tank lots of hits from anything and is incredibly powerful or at least looks like it could crush stuff via mind powers despite looking somewhat fragile (Mew and Celebi for example), has no evolutions, is only found via events or off-the-beaten-path areas that you can only get to in postgame (Things like the legendary birds are fine since they're usually at the end of long dungeons and look imposing), and it has to break the common mold of Pokemon designs and look unique enough to separate itself from the regular riffraff.
Zeroara does not look like a legendary in the slightest, it looks like a Pokemon that would have a Riolu-tier lower form. It's basically gen 7 Lucario/Zoroark but because it only has one form now it's a legendary? Hoopa only looks like a Legendary when in its Unbound form, Meltan/Melmetal don't look mythical, they look like a normal evolution you'd find wandering around a route, Shaymin looks like some random Togepi-tier Pokemon, etc.
To take an example from Digimon it's like they're passing off random Adult-tier Digimon as Royal Knights and you're sitting there like "This is very clearly an Adult Digimon, not something so powerful it protects reality". Regis are great examples of Legendary Pokemon, Dusknoir is another one, the genies are another, Xerneas and Yveltal are another, etc. Zygarde Dog does not look Legendary at all, but everything else that's Zygarde does look legendary.
The problem with giving Legendaries evos is that you've now demoted them to random Pokemon tier. What differentiates Meltan from something like a Golett besides GF calling it a Mythical? Nebby's entire line looks nothing like its final evo, it's not even close, and Nebby itself looks like a random Psychic/Ghost type you could find wandering around a route, not a legendary.