Fish name: Alligator Gar
US Location: Texas, Louisiana, southern states.
Why are they dangerous? Alligator gar have a vicious set of teeth, and put up a vicious fight. They're meat eaters, and are rumored to have attacked humans, though this is really not a confirmed fact. Check out the video on the second image to see what they look like.
Should I be concerned? That depends on if you're fishing for them, or just casually in the water. Alligator gar are rumored to have attacked unprovoked, but nothing is confirmed. Like any other predator of similar size, a chance of them biting exists, but is only likely to happen if they are provoked.
If you're fishing them, be wise: their teeth are seriously share and can really cause some damage. They can also throw a treble off like other big fish, and their motion while trying to land them can be hard to handle as well. Make sure to have buddies, and don't try it without know what you're getting yourself into.
For more dangerous fish: http://www.vingle.net/collections/1128609