Zoroark gets Flamethrower likely because of the kitsune basis.
Hm, how about we quickly go through the Ghost-types from Gen I through VI to see how strong of a theme they had/have:
GEN I:
- Gastly family: The original three. Gastly is seemingly based on a will-o-wisp or the general concept of a formless spirit, combined with gas (possibly a reference to how natural gas is often attributed to will-o-wisp sightings). Haunter is the closest thing we have to a typical bedsheet ghost or ghosts which are disembodied body parts, its Japanese name is even "Ghost". And finally Gengar is a living shadow and its name implies it being a doppelganger, it's noted to resemble a Clefable shadow.
- Misdreavus: Like Haunter, Misdreavus is based on the many disembodied ghosts, most who are flying heads; it could also take inspiration from the banshee with its connection to screaming.
- Shedinja: This is where we start shaking things up. While still a "dead" thing, it's not quite the same way as the Gastly family and Misdreavus which are spirits. Infact, the only way to get a Shedinja is to also get a Ninjask, aka the thing it's supposed to be a spirit of is still alive. It's a discarded exoskeleton husk that somehow got possessed, its hollowness leaving it with 1 HP but unable to be harmed by anything that it isn't weak to (aka would destroy the husk body); and the back where Ninjask crawled out from now sucks out your soul if you look into it. Kind of random tbh.
- Sableye: None of Sableye's possible bases are ghostly spirits. On the surface it just seems like an imp or goblin, specifically fey which are said to live in mines that help/hinder miners digging for valuable minerals such as gems; but this is more of a fairy spirit. It's other basis is the Hopkinsville Goblin which is an alien. It feels like its only Ghost-type because Fairy didn't exist yet.
- Shuppet family: Shuppet is a teru teru bozu, a doll which resembles a bedsheet ghost that's a talisman to bring good weather. Banette is a tsukumogami of a doll, specifically one that was discarded thus has negative feelings to the one who threw it out; it also has some voodoo doll themes. There is certainly ghost themes with both, the teru teru bozu looking like ghosts & the discarded doll gaining a spirit, but both are still very loose reasonings to be Ghost-type if Ghost-types are just supposed to be "dead" things.
- Duskull family: Duskull looks like a little Grim Reaper. Dusclops is a combination of a mummy and another tsukumogami, Chochin-obake, which are one-eyed paper lanterns. The Grim Reaper and mummy connection alone is enough to justify the Ghost-typing.
- Drifloon family: Drifloon is a balloon and Drifblim is a hot-air balloon. Like, what else is there to say? At BEST they're another kind of tsukumogami, like a popped balloon that somehow contained a soul. If you want to go really grim can say Drifblim is based on hot-air balloon disasters (or blimp; don't like you thought about naming your Drifblim "Hindenburg" and the only thing stopping you was you didn't know how to spell it). But, yeah, that's a stretch for it to be Ghost-type.
- Mismagius: So you think if Misdreavus was fine that its evolution would be too... but not really? Like, it barely passes cause it does build upon Misdreavus, but the addition that Mismagius adds is making it into a witch. Certainly spooky, but ghostly?
- Spiritomb: It's not just one soul, but 108 (sinful) souls!
- Dusnkoir: While certainly more stylized, it now has the typical shape of a "wisp" ghost: humanoid but its legs are missing and in their place a "tail"-like wisp. It's also now more like a Grim Reaper as it guides spirits to the afterlife.
- Froslass: Based on a Yuki-onna.
- Rotom: Based on a poltergeist with shades of a gremlin, though it's the poltergeist part that makes it Ghost-type.
- Giratina: The closest you can to it being a Ghost-type if you make it some kind of afterlife god. However, that would be a stretch. It's based on anti-matter, and with the other spacetime dragons represents the gaseous state of matter (Dialga being solid and Palkia being liquid). There's a few mythical connections but none of them really ghost-related: one is a Chinese dragon-eating centipede and there's speculation of it representing fallen angels.
- Yamask family: Yamask is based on Egyptian ideas of a spirit and it's holding a death mask. Cofagrigus is a sarcophagus. Their Signature Ability is also based on the concept of the Pharaoh's curse. So they get a pass.
- Frillish family: Now the funny story with this one is that we know they weren't made to be Ghost-types, they were normal jellyfish Pokemon but they needed more Ghost-types so chose the Frillish family to be changed. This MAYBE had to do with their designs being based on what royalty wore back hundreds of years ago, though for the purpose of this overview that doesn't mean much to justify their Ghost-typing.
- Litwik family: Litwik is a candle, Lampent a lamp, and Chandelure and chandelier. Things you'll certainly find in a spooky house, but is a far cry from the being "dead" or a "spirit" theme.
- Golett family: They're based on both the mythical golem and modern day anime mechs. Really no reason for being Ghost-type. Bulbapedia speculates that they could be based on some terracotta figurines used as funeral objects (or the famous Terracotta Army that was buried along with an emperor), but they're still objects and not "dead" or spirits.
- Honedge family: At BEST they're cursed swords. Bulbapedia also explains there's a Bushido belief about katana containing a samurai's soul. I'll give this one a barely pass as at the very least they say they're possessed swords.
- Phantump family: Possessed tree or at the very least a tree spirit.
- Pumpkaboo family: While it's not a far stretch to connect jack-o-lanterns to spirits, the jack-o-lantern itself usually isn't a spirit and the Pumpkaboo family don't mention they're possessing the pumpkins. They're just random ghost pumpkins.
- Hoopa: The connection between Djinns/Genies and spirits is a bit complex. Like, they're not dead nor have anything to do with souls. They're like fairies in that regard, they're spirits but not ghostly. Heck, when Hoopa changes to its Unbound Form it drops the Ghost-type and replaces it with Dark (so in that regard maybe Hoopa-Confined being a Ghost is an in-lore thing, it's what's left of its true form).
So, yeah, turns out Ghost-types never really had that strong of a "theme" of what makes a Ghost-type a Ghost-type; at least not how you judged what makes a proper Ghost-type. What they did in Alola and doing now with "spooky animals" is just another way they're stretching the theme like they did with Shedinja, objects like the Drifloon family & Litwik family, or using vaguely related mythical creatures like with Sableye & Hoopa.
Until it's hit with an Ice-type move.
Heck, we have a Trainer who went from a Psychic specialist to a Fairy and only needed to change some of their Pokemon of an otherwise "smooth" transition between those types.
And luckily is goroawase number also works for both Psychic- and Fairy-types: 908 which can be read as "kureba" which translates to "clever".
It's a Nightmarcher, ghosts of Hawaiian warriors; so it's a green.
Good catch! So Arezu's Pokemon can pretty much still be anything, or at the very least a Pokemon that gender can be female (which just excludes 17 Pokemon).
Here's my idea for it when it was previously brought up.
