Confusion comes from colonial times. Remember “Florida Panthers?” Florida panthers are not panthers, but cougars. In a similar way, spanish conquerors, which came to the first time to a zoo in the aztec Tenochtitlan, reported seeing “lions, tigers and onzas”. So, cougars, jaguars and some unindentified third felid. It seems that the spanish “onza” was meant to be mythical animal based on an african felid, probably the cheetah.
There are 3 possibilities.
1. There are not onzas but smaller felids like jaguarundis and ocelots (called today by farmers “leoncillo” and “tigrillo”, small lion and small tiger).
2. The onza was a red lynx, which can be found today in North Mexico. This is the most logical explanation for me.
3. The onza was the extinct “american cheetah”. A thin, gracile felid which got extinct during the Ice Ages, which scientist thought it was related with african cheetahs, but now it seems they are related with cougars. Known only by fossils.

I have also seen photos of very unusually colored jaguars, which were incompletely melanistic. They range from almost entirely black with tan or silvery fur near the face, to half-black, half-spotted animals, to animals that were spotted but had a very dark brown base color. I was astonished that such a wide range of melanism exists in jaguars. I always thought that they were either spotted all over, or black all over. Of course I know that the black cats still have the spot pattern, but I had no grasp of the variation that occurs. There are credible reports of albino (or more likely leucistic) jaguars, which have a coloration similar to that of white tiger: bluish-gray or brownish spots on a white background color. I have not been able to find photographs of the white jaguars, but I have seen photos of the oddly colored, incompletely melanistic ones.

Within historical times the jaguar’s range extended into the USA, all the way to the Mississippi Delta (but, surprisingly, apparently not into Florida), quite a way up the Mississippi Valley, and westward to southern California. I think it’s possible that the jaguar may be repopulating some of that range. Recently jaguars were confirmed to be living in extreme southern Texas. Some of the black big cats that are being sighted, at least in the south & southwest, could be jaguars.

South America has some of the least explored wilderness areas on earth. I would not be at all surprised if some previously undescribed species of big cat lives there. With the world warming up, I think it is very possible that some tropical species could be expanding their ranges northward.