Great Auk (Alca impennis or Pinguinus impennis). At 75 centimeters, the flightless Great Auk was the largest of the auks. It was hunted to extinction for food and down for mattresses at least from the 8th century and onward. The last pair were killed July 3, 1844.
They were excellent swimmers, using their wings to swim underwater. Unlike other auks, however, the Great Auk could not fly.
The Great Auk was the original source for the name "penguin". When explorers discovered similar birds in the southern hemisphere, the name was applied to them as well.