>>30724561"legendary" is made up by the fans though.
there is no official categories or lists for what are generally known as legendaries.
there is "mythicals" used for event-mons, which isn't a rule either since Deoxys might very well not be considered mythical now with it being simply obtainable in ORAS like any other legendary.
heck regardless where you ask, most of the fandom cant even agree on a definition for legendary, with everyone just assuming it's obvious. if you check bulbapedias description of "legendary pokemon" for example, it's like listing 3 rules but point out 4 exceptions at every step, because there is nothing that is clearly an exclusive atribute of "legendaries"
the closest we got at the moment to "categories" of what is called legendaries, is the pokedex background colour in Sun/Moon.
regular pokemon=red
600ish TBS "legendaries"=green (tapus, null/silvally, all UBs)
700ish TBS legendaries=yellow (zygarde,cosmog-line,necrozma, magearna..)
Im curious to see if the NatDex in pokebanks update in jan. will have these as well, because I'm wondering if the generic trio-legends will also be green while mascots and most events would be yellow (would be neat if it was more thought through in the sense that Heatran and Volcanion would be te same "category" in that regard for example...)