For mobile users, I am not native but what I learned from duolingo's tips and notes is that there are 2 ways to define things. The first one is to use an "Instrumental case" which changes the form of the object. To define the elephant, you change the object zwierzę into the instrumental case, which has an ending of -em (for masc. noun) / -ą (fem. noun). Sometimes it is changed to -iem ending because of the spelling, which I believe you will see more examples. Another way is to use "'to' + NOMinative case" which creates a sentence like" Tygrys to kot (A tiger is a cat)".