I think something that's important to consider when suggesting retypes is that Game Freak tends to avoid replacing any type except Normal.
True, Rotom's forms did shed Ghost, but they're all alternate forms and Rotom itself is still a Ghost-type Pokémon - I think it's reasonable to draw a line between that and straight-up getting rid of Celebi's Psychic type and leaving no trace of it. (And as much sense as Fairy makes for Celebi, it's also worth noting that Future Sight - a move that has a strong flavor connection to Celebi's time travel powers - is also Psychic, so there's a plausible enough connection there.) No Pokémon
except Rotom, but again, not completely has ever been retconned
out of a type except Normal, and I think Game Freak wanted to keep it that way.
The real problem is the Lake trio, which exist to keep dragons in check and are named after three different kinds of fairies.
As for what the type actually
means... I'm not sure how many people will agree with this, but my interpretation of Fairy includes Pokémon with a few qualities: Pokémon that are
defined by their relationship to humans arguably much more so than most - see Gardevoir, Klefki and Mimikyu for examples, Pokémon that are particularly
connected and attuned to emotions in general
see Togepi, Ralts, Sylveon, Primarina, Cutiefly and Hatterene for examples, and, like
R_N said, a fair few of them end up somewhat
trickstery or two-faced see Mawile, Whimsicott and Grimmsnarl, often with more than one of these qualities at a time. Emotions in particular seem to be a big one
all the more reason why the Lake trio should be Fairy-type....
I might be stretching a bit here - on looking at the actual list, it seems like some of these blur together and some of them might not be as prevalent as I thought - but that's more or less how I've understood the type until now.
It's also pretty common for them to be related to the moon, of course. That's how you get things like Clefairy and Azumarill (moon rabbit), as well as the move Moonblast.
But... for that matter, can we talk about Zacian and Zamazenta? Setting aside whether neither or both deserve to be Fairy-type, what bothers
me about them is that Zacian is somehow
more Fairy-type
than Zamazenta. I'm honestly kind of concerned that Game Freak made it Fairy solely because it's canonically a girl and I resent the implication if so
Honestly, if anything - and I know this is a weird choice, but bear with me - I think I would mind less if they were Fairy/Fighting and Fairy/Steel.
- Fighting is already a type that's been associated with swords as much as Steel (see the Swords of Justice, the move Sacred Sword, and even Sirfetch'd in the very same game), and (seemingly for that reason) Zacian
already has a better Fighting movepool than Zamazenta.
- While it might take a bit of stretching (really only some slight tweaks to their Pokédex entries), I think it can also be argued well enough that Zamazenta is "more Steel than Zacian"
(at least more easily than "more Fighting than Zacian" or that Zacian is "more Fairy than Zamazenta"), since Steel is so consistently associated with defense and armor.
- As for Fairy, any established Fairy-related quality that could possibly be applied to Zacian is
literally exactly the same for Zamazenta:
both are connected to the moon (both naturally learn Howl and Moonblast for primarily flavor reasons),
both conceal themselves in mist (connected to Fairy through Misty Terrain, Galarian Weezing, Slumbering Weald),
both were destined to "slay a dragon" (Eternatus),
both were destined to join with human heroes... What about Zacian sets it apart?
- Aside from that, letting Zacian be Fairy/Fighting allows it to be cool and unique like Game Freak clearly wants
it's so, so obvious that they had a favorite this time around, while letting Zamazenta be Fairy/Steel gives it something it
actually needs in one of the better defensive type combinations in the game.
- also if Zamazenta is the one that's supposed to be a "shield" against Eternatus why is Zacian the one that's immune to both of its STABs
... and okay, ooone more little thing that annoys me. Like I said, it's
way too obvious that Game Freak has a
lot of favoritism towards Zacian. Consider the following:
- Zacian's Crowned Sword form has an incredibly optimized stat distribution, with +10 Speed (giving it an
insanely good Speed tier) and then the remaining 40 added points poured straight into its one important attacking stat. Zamazenta gets -10 Speed (because lowering Speed has a history of making Pokémon better), and since both of its defenses are relevant, it also has to split its stat increases between the two of them.
- Zacian has, statistically speaking, one of the best type combinations in the entire game. Zamazenta actually prefers not to be in its Crowned Shield form because Fighting/Steel is a defensive liability in a metagame surrounded by the specific Ubers Game Freak chose so carefully to carry forward.
- Intrepid Sword gives a free Choice Band-equivalent boost without a Choice lock coming from an already-insane base 170 Attack, so Zacian doesn't even miss any other item for having to hold a Rusted Sword. Zamazenta doesn't even benefit from its Defense boost because it
has no recovery.
- Zacian has access to Sacred Sword, a former signature move that's iconically associated with sword Pokémon and essentially nullifies Dauntless Shield outright (as well as being super effective against Zamazenta). Zamazenta does
not have access to King's Shield, a signature move that's iconically associated with a shield Pokémon and would effectively nullify Intrepid Sword.
- Zacian learns every Fighting move Zamazenta does and also two Fighting moves Zamazenta would die to have. Every Fighting move Zamazenta can learn has some kind of crippling drawback. The main one it runs, Close Combat, literally defeats its own purpose as the shield Pokémon. Meanwhile, Zamazenta does
not get Body Press, the move that was practically made for it-- what, were they afraid it would be broken?
- ...
broken like Zacian?
People in Ubers have actually suggested that Zamazenta would be better off moving down to OU - it's just that OU kinda doesn't want it either. It's the bottom of the barrel of banned Pokémon. And Zacian is seemingly designed to one-up Mega Rayquaza in every possible way.
What were they
thinking with the balance between this duo??
and they even try to go for a climactic showdown between them like Black and White with the final battle against Hop, and it just makes the Sword bias even more comically exaggerated because of how easily Zacian wins no matter which game it is