"An animal is eating cookies."
Translation:Zwierzę je ciasteczka.
"zwierzę" is the basic, Nominative form, used mostly for the subject of the sentence and also in structures like "Kot to zwierzę" (A cat is an animal).
"zwierzęciem" is the Instrumental form, used mostly after the preposition "with" (Polish "z") and in structures like "Kot jest zwierzęciem" (A cat is an animal).
It's the plural of zwierzę.