That Wikipedia entry about fishers makes them sound less menacing than they are, from my understanding. Personally, DH & I & our 70 lb dog were menaced by one once, soon after our cat disappeared. It was pretty fierce, and acted like it was going to come after us. It was in a tree next to our path in the woods, scrambling down to about head-level, and acting vicious, while its young one hung out in another tree nearby. Granted, it did have it's kid with it. Then I started hearing local stories (one that killed a beagle, one that prevented a guy from getting to his house from his woodpile, one that chased a little girl to her house, because she was playing with a kitten and it wanted it, etc). They were reintroduced here years ago, partly because the loggers were having serious porcupine issues, and fishers are about the only thing that messes with them. Apparently they can catch red squirrels running around in the treetops - very agile.

We're in a pretty rural / woodsy / sparsely populated area, with a lot of wildlife. Urbanization was not an issue when we had our problem with them. They just moved in, cleaned out all the squirrels and other prey, and then apparently moved on.