The term 'Fast Flightless Bird' can be summarized by two crossword solutions: RHEA (4 letters) and EMU (3 letters). Rheas are large, flightless birds native to South America that can run at speeds of up to 40 mph. Emus are the second-largest living bird by height and are native to Australia. They are also flightless and can run at speeds of up to 30 mph. Both birds are fast and flightless, making them suitable answers for the term 'Fast Flightless Bird'.