If these black panthers are as large as reported, they could be melanistic jaguars. A few years ago we had a confirmed (normal colored) jaguar here in Arizona. They certainly aren't common but it's possible a few could wind up in other parts of the country. There is only one species of big cat common in N America and as far as I know there are no melanistic mountain Lions.
Jaguars were once found across the Americas as far north as California, Arizona, New Mexico and Texas in the United States (7). In recent years they have been seen and photographed in southern Arizona (6) but otherwise survive only in Central and South America, ranging from Mexico to north Argentina (2). Their population is small and highly fragmented in Mexico. The highest population densities of up to one per 15 square kilometres are found in the lowland rainforests of the Amazon Basin. Although this refuge is of sufficient size to conserve the species for the foreseeable future, jaguar populations are considered to be declining in most other habitats
It's hard to say. Everyone says that before the Mexican gray wolf restocking program that they were extinct in the wild, but I've heard many different unconnected people here say that they have seen them, probably being chased by bigfoot.