Chicken Walks Like A Penguin

Some dictionaries suggest a derivation from Welsh pen "head" and gwyn "white", including the Oxford English Dictionary, the American Heritage Dictionary, the Century Dictionary and Merriam-Webster, on the basis that the name was originally applied to the great auk, which had white spots in front of its eyes (although its head was black).
An alternative etymology, found in a few English dictionaries, links the word to Latin pinguis "fat", from its perceived appearance. This etymology would be improbable if "penguin" were found to have been originally applied to the great auk, as some sources suggest.
A third theory states that the word is an alteration of “pen-wing”, with reference to the rudimentary wings of great auks. This has been criticised for the unexplained nature of the alteration of the word.