"That is my first rhinoceros."
Translation:Dat is mijn eerste neushoorn.
"First" is always translated as "eerste" in Dutch. Adding the '-e' is disregarded for de and het nouns because 'first' is always 'eerste'. The same goes for any time you say second, third, forth, etc. in Dutch. The translation always ends with '-e', no matter the definite article of the noun.
But not the first for Duo.! I still wonder where the Hippos, Zebras, Gorillas, Lions and Giraffes are. To name but a few of the forgotten animals. I can only assume we don't need to know the Dutch word for these. Rhino, Elephant and Duck should be enough for a useful vocabulary. What about Fox, Badger, Squirrel and Seal.?