Blemmyes were an actual African people described in ancient Roman histories who threatened Roman Egypt a few times in the third century AD. The name possibly derives from bálami, meaning “desert people” in the Beja language of Africa. (Note: The proper singular form is blemmyas or blemmye; the plural form is either blemmyae or blemmyes.) Along the way, the Blemmyes also became fictionalized as a tribe of headless humanoids whose faces are located on their torsos.
In various medieval sources, blemmyes are said to be six, eight, or even twelve feet tall and perhaps half as wide. Furthermore, they are often reported to be cannibals.
Herodotus described such creatures in the fifth century BC, calling them akephaloi (“headless ones”). How a real-live human population came to be seen as headless monstrosities is anybody’s guess. Certainly the nearly universal human tendency to demonize and dehumanize one’s enemies is at play. More concretely, some propose that the historical Blemmyes had an unusual fighting stance that involved tucking the head close to the chest, or else they had the ability to raise their shoulders to an extraordinary height, nesting their head in between. Others ponder whether these reports of “headless giants” involved the custom of painting faces on their shields.
At any rate, the blemmyes as a “wondrous tribe” apparently captured the imagination of ancient geographers and naturalists, not to mention the centuries of learned Europeans who had studied their tales. In time, the legend of these headless men shifted from Africa to India. From there, as with so many ancient wonders, they found their way to the New World just in time to be discovered by European explorers.
In 1595, Sir Walter Raleigh reported that, along the Caora river, there lived
a nation of people whose heads appear not above their shoulders which, though it may be thought a mere fable, yet for mine own part I am resolved it is true because every child in the provinces of Arromaia and Canuri affirm the same. They are called Ewaipanoma. They are reported to have their eyes in their shoulders and their mouths in the middle of their breasts, and that a long train of hair groweth backward between their shoulders. (Discovery of Guiana, spelling and punctuation updated)

Section of the Piri Reis map depicting a blemmye and a monkey
Raleigh reported that these blemmyes lived in the same general area as the fabled city of Manoa, which the Spanish called El Dorado.
But Raleigh wasn’t the first person to “find” blemmyes in the Americas. Nearly a hundred years previously, Ottoman admiral and cartographer Piri Reis depicted a blemmye in South America (near the coast of Brazil, to be specific) on his world map of 1513. Beside the drawing, he explains, “These wild beasts attain a length of seven spans [5′ 3″]. Between their eyes there is a distance of only one span [9″]. Yet it is said, they are harmless souls.” Thus, these blemmyes are a fair bit smaller than their Old World cousins—and apparently less hostile to humans.
When William Shakespeare wrote Othello in 1604, he included this fabled tribe among the oddities his title character reports as he describes his earlier exploits:
And portance in my travels’ history:
Wherein of antres vast and deserts idle,
Rough quarries, rocks and hills whose heads touch heaven
It was my hint to speak,—such was the process;
And of the Cannibals that each other eat,
The Anthropophagi and men whose heads
Do grow beneath their shoulders. (Act I, scene 3)
I should also note that the Hidatsa, a Siouan people from present-day Minnesota, have a myth about a “headless monster” with a gaping mouth in its shoulder. who kills the mother of the Twins, important culture heroes. I have no way of knowing if those who tell this story imagine a blemmye-like creature with eyes on its chest or shoulders, or else something else entirely. In any case, there are a few headless monsters in world mythology: in addition to the blemmyes and their New World kin, there is also the dullahan of Ireland, a headless horseman immortalized for Americans in Washington Irving’s The Legend of Sleepy Hollow.
I don’t want to think about blemmyes riding around on horses. Though perhaps the ones in Guiana could domesticate some Brazilian headless mules to ride.