Well, there are dogs, foxes, jackals and wolves

They can be used in many ways for supernatural beings

A bit out there, but I sometimes thought of those deep-sea translucent fishes as kinda ghost-like. That or perhaps jellyfish. I suppose an underwater-themed Halloween picture would be pretty unique though! ^_^

Argh I've had another real tricky idea for jellyfish inspired cha before, but couldnt settle on that design either, might be worth taking another crack at it.

Spiders! Maybe fishmen for the swamp-thing vibe, flies for the The Fly style mad scientist vibe, Frankenstein-y patchwork creatures..?

Thinking what vibes the most with ghoooooooosts?

I totally missed that, woops! Maybe worms or pale moths? Snails for that ectoplasmy goopy vibe? Possessed plush bears?

Ravens :>

Dogs and canines of most types are a common thing, at least spirit wise, and also blackbirds! (Crows and ravens)

Obviously wolves, because werewolves.

Frogs and toads work due to their association with witches, and specifically their association with being what someone ends up as if they anger a witch. Also because the Swamp Thing is a classic movie monster.

Spiders are another Halloween staple, you can't go anywhere in the month of October without seeing those big, fake cobwebs everyone puts on their houses to give them that spooky, abandoned feel.

Foxes aren't really considered Halloween-y by modern standards, but they're often associated with trickster spirits so they'd be a good fit.

Crows and Ravens, of course, are often seen as heralds of death, and can frequently be spotted in large flocks in the fall.

Owls are another bird that tend to be more noticeable in fall, and the hooting of some larger owls is very somber and chilling.

And of course you can make any mythical monster fit Halloween.

Finally, I know this is a bit of a stretch, but any sort of carnivorous plant amped up to 11 can work.

With ghosts and spirits?

Black dogs
Pale horses
Owls
Vultures

I'll keep poking around and see if I can come up with some more..

Inexplicably, I think of all those old folk tales with ghost horses, not to mention dullahans; so dullahan equitaur/anthro-horse?

The only thing I can think of are the white stags that appear as aethereal and otherworldly apparitions in the folk lore of certain regions.
A majestic and ominous spectral deer would be an interesting concept, for sure!

I gotta draw my plant-deer macro again for autumn, might lean in that direction, but not quite sold on it as ghastly....