An image purportedly showing a strange creature that was terrorizing local residents as it roamed the streets of a small village in Argentina popped up on various conspiracy web sites and YouTube channels in early April 2018:
The creature's shape is striking, as it has not been connected with any known animal. Residents of Totoras in the province of Santa Fe report that a creature is wandering the streets and has even slain two dogs.
Although this image was initially confined to web sites dedicated to aliens, UFOs and other "unexplained" mysteries, it also received coverage from more mainstream outlets such as The Mirror, where it was presented as if it were a genuine news item:\
Monsters that roam the night are a myth, right? Demons only exist in Harry Potter books, don't they? So explain this terrifying footage of a mysterious creature terrorising a neighbourhood.
The giant animal has so far butchered two dogs — a German Shepherd and a Pitbull.
The pictured creature is not terrorizing a neighborhood in Argentina, nor did it slaughter any pet dogs. Why? Because this demon is fictional, existing only within the fantasy universe of the Harry Potter series of books.
This image was created by manipulating a piece of concept art for Remus Lupin, a professor afflicted by Lycanthropy, as depicted in the work Harry Potter and the Prisoner of Azkaban. The image is available via the Pottermore web site, which also offers more backstory about the werewolf professor:
It is established in Harry Potter and the Prisoner of Azkaban that Remus Lupin, while a child, was bitten by the werewolf Fenrir Greyback. As an adult, he is able to take Wolfsbane Potion to control his condition, but he still struggles with the shame and weight of his secret, which the filmmakers brought out through costume and makeup.
Here's a comparison of the Harry Potter concept art and the "mysterious" creature supposedly roaming Argentina (with the concept art flipped for a better comparison):