I don't think it's going to be a water-type for two reasons.
First, if it was a kelpie, a lake spirit, you'd think the nature-camera probably would have been in a lakeside setting. Instead we got it in the middle of an enchanted forest with not a single lake or stream in view. It is the home of bioluminescent mushrooms and floating, spritely orbs, which is exactly where'd you'd expect to find a lot of fairies.
Secondly, if it was a water-type, I think they would have made it just a little more obvious in it's design. Its mane looks like a fluffy, purpish cloud. If it was a water-type, I would expect its mane to appear more like seafoam. I'll give credit to the theory where it's due though, that greenish bit on its mane does somewhat resemble the color of an algea-filled lake, but given everything else I don't think that's enough to hint at it being water type.
On that note, I think it's going to ditch its counterpart's fire-type as well. If it was still fire-type, you'd think they would have just given it pink or purple flames instead. The only thing linking Pontya's design to fire has been replaced in it's Galarian form.
I'm going to make a hard read and say it's pure fairy. It could maybe it could be fairy/ghost because of its ghostly purple mane but I don't think it's spoopy enough for that. And it's not like fairy-types haven't been associated with mists in the past.
Edit: Adding a third observation, kelpies aren't usually depicted as having horns, while unicorns are practically defined by them. Adding a horn to Ponyta implies that they're giving a greater emphasis to the Galarian line's horns, which would make more sense if they're more unicorn-inspired then kelpie-inspired.