Which Generation VIII Pokemon is a Ghost type, Polteageist or Sizzlipede?

Between Polteageist and Sizzlipede, the Generation VIII Pokémon that is a Ghost type is Polteageist.

Polteageist is a Ghost-type Pokémon introduced in Pokémon Sword and Shield, the main-series games that began Generation VIII. It resembles a haunted teapot filled with tea and evolves from Sinistea when the appropriate evolution item is used. Polteageist has a distinctive relationship with its container: its species can exist in either a counterfeit or authentic form depending on the Sinistea it evolved from.

Sizzlipede is the contrasting choice in this question. It is a Fire/Bug-type Pokémon, not a Ghost type, and it evolves into Centiskorch. Polteageist is also easy to confuse with Poltchageist, a later Pokémon introduced in Generation IX; Poltchageist is Grass/Ghost and is not an evolution of Polteageist. Thus, among the two named Generation VIII Pokémon, Polteageist is the single canonical answer.