The onza is a type of South American cat. It was first described by conquistadors as a species distinct from the two well-known large cats of South America, the cougar and the jaguar. The name came from the Latin word Uncia, which in those days often referred to the cheetah, since the conquistadors believed that it was a form of cheetah. Nowadays, Uncia more frequently refers to the snow leopard (Panthera uncia).
The onza was described as being longer than the cougar, but longer-legged and much more lightly built. It was often described as having pointed ears, which is typical of most smaller cats but not of the cougar or jaguar. Locals feared the cat and considered it fierce. They were quite adamant, as well, that it was a different animal from the well-known cougar.
The scientific community showed no interest in investigating these reports, ridiculing the whole thing as superstition and cryptozoological humbuggery. Even when a hunter shot an onza in 1938, no scientific examination was conducted. Luckily, the hunter kept the skull. In 1986, another one was killed by a farmer in Mexico, who believed he was being attacked by a jaguar. He contacted a nearby experienced hunter for advice, though, when he discovered that the animal he had killed seemed to be neither a puma nor a jaguar. The carcass was then taken to a university in Mazatlan for further study.
In 1998, researchers at Texas Tech University examined a frozen corpse of an onza, possibly the same one shot in 1986. They did some DNA tests, and reached the conclusion that the animal, while physically distinct, was genetically indistinguishable from an ordinary Puma concolor. They did not rule out the possibility that it was a previously unrecognized subspecies, however.
Some reports state that the specimen examined at Texas Tech had non-retractable claws, though I have not been able to determine the original source for this claim. This would be unusual, since most cougars have retractable claws, like all cats except the cheetah. German zoologist Helmut Hemmer suggested that the onza may represent a relict population of the American cheetah Miracinonyx trumanii, which would explain the non-retractable claws, but comparison of known fossil M. trumanii skulls with the skull of the onza confirms that they are different animals. Recently discovered DNA evidence sheds some light on this, though: it turns out that both modern cheetahs and the extinct M. trumanii are fairly closely related to the Puma genus. It is therefore possible that the gene for non-retractable claws exists within the cougar genome, and may have surfaced due to random mutation, either as a common trait in a subspecies, or as a freak mutation in a single aberrant individual.
The mystery of the onza isn't entirely solved, but it's a lot clearer than it was. We now know with reasonable certainty that a population of oddly-colored cougars, either a color variant or a proper subspecies, exist in South America. They remain unstudied in the wild, and only minimally studied at all. Some cryptozoologists hold out hope that the onza will eventually be declared a separate species, though without further evidence, this seems improbable.