The Stolzmann's fish-eating rat was one of the rarest mammals in the world, known from only seven specimens.
But a new survey has revealed that, far from being on the brink of extinction, this once enigmatic rat species is becoming a "nuisance" to local fishermen, stealing their catches.
The rise of the rare rat may be due to the increase in trout-fishing in South America, where the rat lives, providing easy meals for a species that locals now see as a pest alongside other rodents, parrots and pigeons.
Details of the rat's new status are published in the journal Mammalian Biology.
Stolzmann's fish-eating rat (Ichthyomys stolzmanni) is a medium-sized rodent measuring around 20cm from head to tail.
It is one of the least known rodents.
Before the new survey, it had only been recorded seven times.
The rat was first discovered in 1893 at an altitude of over 900m in Chanchamayo, near Tarma in Peru.
Six further specimens were found in the 1920s at a similar altitude, but this time in Ecuador.
The Stolzmann's fish-eating rat was not then recorded by scientists in the wild for a further 90 years.
That was until Peruvian biologists Victor Pacheco and Joaquin Ugarte-Nunez conducted a new survey, searching for the species.
They managed to trap four new specimens of Stolzmann's fish-eating rat in the department of Ayacucho, Peru, more than 300km further south than where the species was originally found.
But they also uncovered evidence that suggests this rat species is far more ubiquitous than previously thought.
Indeed, they go as far as to say that the Stolzmann's fish-eating rat is becoming a "nuisance".
Local people, landowners and workers at fish farms along the Vinchos river in Ayacucho told the scientists that they regularly see the rats scurrying around the town of Vinchos, and that sightings have increased since the 1970s, when trout farms in the area expanded.
The locals also told the researchers that Stolzmann's fish-eating rats regularly eat baby trout being grown in the trout farms, reducing the farm's productivity.
Fish farm workers try to trap the rats, often without success, and try to hit them with sticks or capture them with fishing nets.
But they report that the rats usually escape, as they are very fast and agile swimmers.
Villagers consider the rat to be such a nuisance that they call them "mayo sonso", a derogative name that applies to a riverine animal that causes damage, and that they have tried to eradicate the rodents by burning grassland at the edge of the Vinchos river, hoping to destroy habitat where the rats live and reproduce.
"It is a surprise to find what you consider a rare species is not anymore," Dr Pacheco told the BBC.
The scientists do not think there has been a sudden explosion in numbers of the fish-eating rats.
"Because of the increase of trout farms the rats are more ubiquitous. I do not think [their numbers] are rising rapidly," says Dr Pacheco of the Museum of Natural History in Apartado, Lima, Peru.
But "it surprises me how easily they are adapting to man-made changes in the environment."
He believes so few Stolzmann's fish-eating rats have previously been found because the rodent is so difficult to trap.
It could also be that previous surveys were looking in the wrong place.
Despite the rise in Stolzmann's fish-eating rat numbers around trout farms, the species may remain at risk.
Little is still known about it, and like other aquatic species, it may be threatened by deforestation and contamination and pollution of the streams in which it lives.