Above is a series of images from a book by Dutch traveller and explorer Jan Huyghen van Linschoten [1563-1611], which depicts some natives and weird animals found at the Strait of Magellan. In 1598 he piloted the Dutch fleet on its first voyage by the South-West Passage (of Magellan's Straits) to India, so he really saw what is depicted in his book.
It is remarkable because the upper part shows a seal-like animal with a long thin tail and two sharp walrus-like fangs jutting out of its mouth. The caption says "animal of the Strait of Magellan".
The central part shows a man and a woman which are described as "Magellaneis" (native to Magellan's Strait).
The bottom part of the image shows two men kneeling in front of an idol with horns and snakes protruding from its head. The caption is quite illegible, and I am not sure it shows Fuegians or Aonikenk natives because it seems to say "Lapons" (Laps), and Linschoten did sail to Lapland (northern Scandinavia and Russia) in the late 1590s. So maybe they are not Patagonian natives.
Walruses (Odobenus rosmarus) are animals that live in the Arctic areas of Asia and America, along its northernmost reaches, in the Arctic Sea and also in the Atlantic and Pacific Oceans. Its long tusks and massive body are the two features which distinguish it.
They evolved in the North Pacific some 18 Million years ago and may have extended their range to Northern Mexico and California. Some 5 to 8 Million years ago, they spread into the Atlantic Ocean through the Central American Seaway, which was a channel that linked the Pacific and Atlantic Oceans when North and South America had not yet joined. 
Could these primitive odobenids have moved into the Southern Hemisphere colonizing its coasts with the other southern seals? I have not found any references mentioning walruses in the South Atlantic, or outside of its current northern circumpolar habitat.
There have been historical records which report sightings of walruses in southern England, Ireland, Germany, Netherland, Spain, Belgium and New England (U.S.). So they could have drifted southwards across the Equator into the South Atlantic.
There may be proof of this in old accounts about Patagonian "Horned" sea creatures, which I had assumed were descriptions of terrestrial cow or bull-like creatures frolicking in the sea. But they may actually describe a walrus-like sea mammal (if instead of "horns" we take them for "tusks"). These animals were reported in Chiloé in Northern Patagonia and also in Tierra del Fuego (right beside the Strait of Magellan). Perhaps they do refer to the same creature, which nowadays is probably extinct.
Yet there was a strange whale or dolphin-like creature that looked like a walrus which lived in the Southern Hemisphere about 4 or 5 Million years ago, the Obenocetops peruvianus. Its remains have been found in Peru (in the Pisco Formation of Early Pliocene age), and the animal's reconstruction is strikingly similar to a walrus:
Another very similar species was discovered also by Muzion in 1999 , in the same area, and the same period but is about 1 Million years younger, the Odobenocetops leptodon (its image can be seen below):
Its tusks point backwards instead of downwards as in walruses.
Note that none of these animals have the snake-like tail of the "Fuegian Walrus" shown in the first image.
 Memoria Chilena, Portal de la Cultura de Chile. Histoires de la navigations. Jean Hugues de Linschot .../ avec annottations de B. Laludanus. 3eme ed. augm. 1689. 522 p.,  h. with plates.
 Annalisa Berta, James L. Sumich, Kit M. Kovacs, (2006). Marine mammals: evolutionary biology. Academic Press. pp. 124.
 Ronald M. Nowak, (1999). Walker's mammals of the world. JHU press, vol.1, pp. 862.
 Christian de Muizon, (1993). Odobenocetops Peruvianus: Una Remarcable Convergencia De Adaptación Alimentaria Entre Morsa Y Delfín. Bull. Inst. fr. études andines 1993, 22 (3): 671-683
 Muizon, C. de, D. P. Domning & M. Parrish, (1999). Dimorphic tusas and adaptive strategies in a new species of walrus-like dolphin (Odobenocetopsidae) from the Pliocene of Peru. Comptes-rendus de l’Academie des Sciences, Paris, Sciences de la Terre et des Planetes 329:449-455.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©