YOU SAID:

I have a squirrels in my underwear

INTO JAPANESE

下着にリスがいる

BACK INTO ENGLISH

There's a squirrel in my underwear.

INTO JAPANESE

私の下着の中にリスがいます。

BACK INTO ENGLISH

There is a squirrel in my underwear.

INTO JAPANESE

私の下着にリスがいます。

BACK INTO ENGLISH

There is a squirrel in my underwear.