Jagurundi's can be fairly dark, but they are small and rare, and even more shy than lions.
I thought I saw a black one in northern AZ some years ago; just a glimpse of the body and a long tail. I was told by a big cat sanctuary owner, though, the same - no one has ever documented a black mountain lion. If they do exist, they must be as rare as white buffalo, even though jags and leopards do come in black. Heck, maybe they are escaped, captive black leopards?
Supposedly there are lions on the new deer lease. Guess I'll have to be careful...
