There’s universal agreement today that a penguin is:
Any of various stout, flightless aquatic birds of the family Spheniscidae, of the Southern Hemisphere, having flipperlike wings and webbed feet adapted for swimming and diving, short scalelike feathers, and white underparts with a dark back (AHD).
It’s possible that penguin is of Welsh origin; it breaks down very neatly into pen + gwen/gwyn, with pen meaning “head,” and gwen meaning “white,” (and there are species of penguin with white heads).
However, the etymology isn’t at all certain. The OED offers two early quotations in context:
1577 F. FLETCHER Log of ‘Golden Hind’ 24 Aug. in N. M. Penzer World Encompassed (1971) 128 Infinite were the Numbers of the foule, wch the Welsh men name Pengwin & Maglanus tearmed them Geese.
1589 N. H. in R. Hakluyt Princ. Navigations (1589) III. 809 The Port of Desire… In this place we had gulles, puets, penguyns, and seales in aboundance.
The OED also expounds in a lengthy etymology note after first observing that penguin was first applied to the Great Auk, and that penguins resemble the Great Auk as a large flightless waterfowl, with black-and-white as primary colorations:
The attribution of the name penguin to ‘the Welsh men’ . . . and its explanation as Welsh pen gwyn white head, appears also in Ingram’s Narrative, and later in Sir Thomas Herbert’s Travels (in the edition of 1634 as a surmise, and in the edition of 1638 as an accepted fact). Since the bird was known in the far north of Europe under a different name (see GARE-FOWL n.), it is likely that the term penguin originated in North America. However, the Great Auk did not have a white head (though it had large white spots in front of the eyes).
The OED notes in closing that
An alternative derivation of the name < classical Latin pinguis fat (see pinguid adj.) or an early association with this word is therefore possible and may be supported by the relative frequency of forms in pin– in most languages from an early date. Compare German Fettgans fat goose, also penguin (18th cent.).
Nonetheless, a Welsh influence, if not a derivation, does seem to be a reasonable explanation for penguin.