Questions & Answers about Mój pies jest chory.
Polish possessive pronouns change form to agree with the gender and number of the noun they describe, not with the owner.
- mój – for masculine singular nouns in the nominative (e.g. mój pies, mój kot)
- moja – for feminine singular nouns in the nominative (e.g. moja mama, moja siostra)
- moje – for neuter singular nouns (e.g. moje dziecko) and some plural cases
Since pies (dog) is grammatically masculine singular, you must use mój.