"Big fish eat small ones."
Translation:Velké ryby žerou malé.
Theoretically yes. It could mean that the small ones of something (of the feminine gender) eat big fish. But it would be a very confusing way to say that because everyone would first understand the meaning presented here. Maybe in some very special context which is hard to imagine now.
Different grammatical cases and numbers. Please read the Tips and notes provided first, we cannot repeat it all here. Use your web browser (mobile or PC) to access them. Click on "TIPS" or the lightbulb when selecting the skill to get to https://www.duolingo.com/skill/cs/Animals/tips-and-notes