What I think the distinction between 'Legendary' and 'Mythical' is.

Legends are stories based on historical events, usually true stories but are almost always embellished. So legendary Pokémon are just that, Pokémon known to exist but are still surrounded by mystery. Often revered, feared, and in general hold a deep connection and purpose to the lives of people.

Myths are stories that are wholly fantastical, symbolic, sacred, and considered to be fictional. That's what mythicals are. Fantastical creatures that are rumoured to exist but whose actual existence is unknown. Encounters and stories involving them are scarce and many might not believe them.

This is also why:
- many legendary Pokémon are implied to not be one of a kind, while for mythicals, only one of them exist. (Usually. I'm looking at you Meltan.)
- why legendaries count for the pokedex and are required for dex completion while mythicals are not. They aren't even sure they even exist.

There's probably some legendaries or mythicals that break this archetype a bit so it's not foolproof but I think this serves as a good basis.