There are a wide range of mystery carnivores reported in North America. Strange felines are most common (black, striped, spotted, maned, or brindle), but there are also reports of hyena-like animals, oversized polar bears, miniature grizzlies, huge white wolves, and more.
Varmints explores the diversity of sightings and folklore of unknown predators, and also examines what we know about the oddities (phenotypic and behavioral) of our known carnivore species.
There is a lot of material here for both the beginning cryptozoology enthusiast and the seasoned researcher. With reports spanning North America, there are plenty of leads for future investigation.