"You see a big animal."
Translation:Widzicie duże zwierzę.
because kot , and pies are masculine animated nouns, and zwierzę is neuter noun.
Masculine nouns (in singular), can change in accusative case or not depending of their "animacy" (animated nouns usually have accusative=genitive, not animated nouns have accusative=nominative)
Neuter nouns have always have accusative=nominative.
Feminine nouns (in singular), have accusative form that is different form other cases.