It’s not showing on the website at this point in time and I want to know
You can find Poltchageist in: Three-Star Tera Raids in the Mossfell Confluence Reveler's Road All these locations are only in the DLC. You can also trade from another person who has the Pokémon you request. Note: You can only trade Poltchageist to your game if you have Version / Update 2.0.1+ of the game
There's also a fixed Tera Water Poltchageist in Mossfell Confluence.
This is also true.
Source (Poltchageist game locations)