King-weasel
(Also "Arctegemon" or "Slashbear") Macrogulo gulo: the large and ferocious antecedent species of the present-day wolverine, Gulo gulo. Some theorists suggest that the King-weasel may be a relict species of the Pleistocene mustelid ancestor Plesiogulo.
King-weasels were the natural enemy of the fireworm and numerous other creatures exported to North America, South America and Europe in the aphthonic intervention project. Omnivores with large territories, they routinely grew to the size of the black bear, and readily competed with bears for food all along their northern range.
The king-weasel became extinct in North America in the late 1800s due to early industrial pollution and encroachment by humans on its breeding and hunting territories. However, before its extinction, the species hybridized readily with Gulo gulo, and members of that species are now the only North American carriers of the king-weasel's genetic heritage. There are reports in the mountains of Chile and the high plains country of Argentina that describe creatures possibly descended from the king-weasel.
Otherwise, the king-weasel is something you really don't want to meet in a dark alley. Or anywhere else. Really not.
(see also: Cockatrice: Fireworm.)