"sterk" is being used as a predicate adjective.
Here's how adjectives are declined:
For definite articles, the adjectives always get an "e" ending.
- De sterke neushoorn
- Het sterke dier
- De sterke neushoorns
For indefinte articles it depends on the gender.
- Een sterke neushoorn. (because it's "de neushoorn")
- Een sterk dier (because it's "het dier")
Then if the adjective doesn't come right before the noun it modifies, it's known as a predicate adjective and doesn't get declined.
- De neushoorn is sterk.
- Het dier is sterk.