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Elëngassën is a humanoid monster reported from Patagonia The first one to report it was a Swiss rancher and naturalist, Jorge Claraz (1832-1930), who in 1866 set out to explore the interior of the territories of Rio Negro and Chubut. In his diary he rendered the natives, their language, and their customs. He also mentioned an “Elengassem[*] cave” close to Segunda Angostura, the second “narrows” of the Negro River, near to what is now the town of Guardia Mitre in Rio Negro province, Argentina.

This cave was once inhabited by a strange creature, the Elëngassën, which he described as:

"An animal similar to a man -it has a human figure- but is very big. It has hands, big legs, it walks like a man and is covered like a peludo[#] with an enormous hard shell -which is of stone- these beings existed before, but now they are extinct. They were harmless and never attacked. But when one came near them -especially at dusk- they threw stones. These strange beings lived in caves.

He visited the site, but it had caved in and was filled with debris. He could not verify if it held any bones or native paintings. It was set in a very strategic location, in the narrows, above a “very ancient road, that was abandoned because of Elemgassen. It was not possible to pass along it at night […] without being bothered terribly by the animal that threw stones, which is why they made a road above it."

Ten years later old Tehuelche chief Sinchel showed Francisco Moreno this cave stating that it was the “lair of one of these monsters”, and he too noted that “to avoid meeting it they had made a very difficult road with a detour of nearly one league [5 km or 3.1 mi.] over a hill”. The new path was made by “the women […] who were afraid of the animal because it threw stones at them and it ‘growled ugly’ insulting them.”

Francisco Pascasio Moreno (1852-1919). Argentine scientist. He explored the Patagonia several times in the 1870s, being named in 1877 Director of the Buenos Aires Anthropological Museum, which in 1884 became the La Plata Natural History Museum. He is usually known as “Perito” Moreno (Perito means "specialist, expert") for his key role during the border conflict between Argentina and Chile from 1892 to 1902 which defused an inevitable war. He donated a vast tract of land that the Argentine government gave him as a reward for his services and formed Argentina’s first National Park. He served as a member of Congress.

This was not the only one; the beast had lived in several places. Claraz mentioned that “Éllengassen’s most beautiful home” was located at Yaulemtzca –close to current Llama Niyeo (41°54’ S, 68°24’ W). He visited it and dug there finding some bones and ancient rock art.[8] Moreno also found many other caves that the natives believed were “anciently inhabited by the Elengassen.