Paladin of the Snow Queen
And Palossand, assuming it doesn't debut in this episode.
It's too early for Acerola to appear.
I actually first thought haunted house episode because we haven't had one of those yet, but I highly doubt it. Sandygast seems like a weird Pokemon to have appear in a haunted house.
But what if it's a giant Palossand that itself is the haunted house?