The old Egyptians believed that men had souls, which they called "Ba" which after death, could move between our world and the world of the dead.
They depicted it as a "human-headed hawk (see following image), which shows "Ba" visiting the dead man.
Our Greek legacy
The Greek took this same creature and distorted it, from being a good and caring entity, it became an evil creature.
For the ancient Greeks, Harpies were invariably winged women, a "bird-woman" who originally were a "group of wind deities".
Their name, derived from Greek "Αρεπυια" (snatcher), implies that "they are the Snatchers [...] women-demons [...] carrying all things to destruction"
They not only snatched souls and took them to death, they also brought forth life, making mares pregnant. 
The following image shows the famous "The Harpy tomb", ca. 470 BC, in the British Museum. The creature is carrying off a baby.
These monsters with faces of maidens had, according to Virgil, obscene habits. They were ravenous and stole the food from Aeneas at the island of Strophades on his long journey from Troy to Italy.
Later Greeks transformed the Harpy into the "Siren", which can be seen in depictions of Odysseus on his long trip home from Troy.
South American Harpies
South America also has its harpies; the American Harpy Eagle (Harpia harpyja), which lives in the Tropical and Subtropical areas of South and Central America, from southern Mexico to northern Argentina and the south of Brazil.
It is the only member of the genus Harpia (besides the mythical Harpy) and it is the largest of all eagles (with a wingspan of 2.4 m - 8 ft.).
But, there are accounts of Harpies (the Greek kind) in Chile, quite close to Patagonia (For the full text in Spanish, see reference  below).
This alleged discovery of a harpy in Chile in 1784, led to a fashion, and the "harpy costume" in France during the reign of (later guillotined) King Louis XVI. 
The following is the text (in French) 
The context of the 1784 harpy.
These were revolutionary days, the Female Harppy found in the "Royal Province of Chile" was bestial and had mixed genders: she/he was mustached and bearded. Her breasts were surrounded by hair.
The beast feasted on eels, fish and a daily sheep. The first report was a pamphlet by the Count of Provence (Description historique d'un monstrue symbolique pris vivan sur les bords du lac Fagua [*], pres de Santa-Fé, par les soins de Francisco Xaviero de Meunrios, Comte de Barcelone.).
This pamphlet intended to mock the royal family and its lavish ways. So it is not surprising that the beast took on a more femenine appearance to resemble Queen Marie-Antoniette (hated by the populace and depicted as a harpy).
The beast was thus an act of pre-French Revolution hatred to the royal family and not a real Chilean entity.
Tagua Tagua lagoon monster
It was placed  in Lagoon Tagua-Tagua (which now does not exist, as it was dessicated in the 1830s). Don't confuse it with the Lake Tagua-Tagua in the Reloncavi Fjord area which is also home to a lake monster (one was a lagoon, the other is a lake).
Charles Darwin visited it in 1834 before it disappeared, and who wrote about it in his Beagle diary:
The lagoon was about 10 km (6 mi) from the town of San Vicente de Tagua-Tagua in the Chilean province of Cachapoal. Egg shaped, it measured about 10 km long and 13 km (8 mi.) It was shallow (only 5 m deep - 16 ft.).
Chilean author, Orestes Plath  mentions a monster in this lagoon: an aquatic winged being with two tails and scales which carries away the cattle.
Surprisingly, a Patagonian monster, the Llaima Volcano monster resembles a harpy, clawed, bird like and with a human face it is harpy-like.
Tagua Tagua is also one of the oldest sites archaeological sites in America, with clear indication of human activity dated to about 12,000 years ago.
 The Journal of Hellenic studies. (1893). London: Soc. for the Promotion of Hellenic Studies. v.13, pp. xlii and 106.
 Harrison, J., (1991) Prolegomena to the study of Greek Religion. Princeton Univ. Press. pp. 101+
 Edwards, A., (1891). Pharaohs Fellahs and Explorers. New York: Harper & Brothers. pp. 187+
 Picasso, F. (2006). Estudio sobre las supuestas arpias capturadas en Chile (1784) y Perú ( 1829).
 Augustin Challamel, John Lillie. (1882). The history of fashion in France: or, The dress of women from the Gallo-Roman period to the present time. Low, Marston, Searle, & Rivington. pp.174
 Hatin, Eugene, (1864). Histoire politique et littéraire de la presse en France: avec une introd. historique sur les origines du journal et la bibliographie générale des journaux depuis leur origine. Poulet-Malassis et de Broise, v.8. pp. 104+
 Landes, J. (2003). Visualizing the Nation: Gender, Representation, and Revolution in Eighteenth-Century France. Cornell University Press. pp. 215.
 Plath, O., (1994). Geografía del mito y la leyenda chilenos. Grijalbo. pp. 124.
 Weber, G. Laguna Tagua Tagua site (Liberador, Chile) Online.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©