Osriches don't fly becuase they are that big. But poor example.
I see what you mean though becuase most pokemon are infact based on normal animals. But at the end of the day most are just to big to be normal animals (like most of the bug pokemon can't exist because there isn't enough oxygen in the atmosphere) and have fucking attacks like flamethower and ice beam. Tell me the last time you saw a fish shoot a beam of ice out of its mouth.