The format of this question was to fill in the blank with the "correct" Polish word. However, I was asked to fill in for the word dog. Since "kota" is singluar, and "mamy" just means "we have" (or so I think), I thought "Oh, the answer must be the singular version,". However, my answer was marked incorrect and I was told the correct answer was the plural version of "pies". I'm confused now. Does "mamy" imply that "pies" should be plural, or is there something else I'm missing? As of now, I think "pies" and "psy/psem" could work.
And "Mam pies" is definitely incorrect. "pies" is a masculine animate noun, and for those, the Accusative form is identical to the Genitive one, therefore "Mam psa" and "Nie mam psa" both use 'psa'.
Unlike let's say "stół" (a table), which is inanimate and the Accusative form is identical to the Nominative one. "Mam stół", but "Nie mam stołu".