Technically, black panthers do not exist.
The (black) panther is a melanistic variant of other cats, like the jaguar, cougar, or leopard. Close examination of the color of panthers will show that the typical markings of these other cats are still present, but are hidden by the excess black pigment melanin, giving an effect similar to that of printed silk. This is called “ghost striping”.
This means that, technically, a panther is really a jaguar or leopard who has a black coat; what we call panthers are really other species of big cats with a different coloring scheme. The panther as a separate species has never been scientifically documented.
Because panters are simply color variants on other big cats, there are numerous reports indicating that the “real” black panther is a mythical creature, and has become a subject of cryptozoology (a field including the perennial Bigfoot).