YOU SAID:

There's a pokémon joining in my house

INTO JAPANESE

家にポケモンが入ってきた

BACK INTO ENGLISH

There's a Pokémon in the house.

INTO JAPANESE

家の中にポケモンがいる。

BACK INTO ENGLISH

There are Pokémon in the house.

INTO JAPANESE

家にはポケモンがいる。

BACK INTO ENGLISH

There are Pokémon in the house.