I "think" it might be some kind of dragon-fly or caddis-fly larva. (not an expert here).
I base this on seeing similar beasts when in my early years I identified similar creatures whilst dipping in ponds when I was at school. I actually found something similar last year when I got my plants into my tank, the creature had made a little tunnel nest out of bits of plant and was sitting at the water's surface in the plants in the tube it'd attached to the plants. When I got it out when pruning the plants the little guy legged it, it was pretty vicious looking and I think when they get larger they can sometimes predate on small fish.
I "think" they do breathe air, through their tail I think (but I'm pretty fuzzy on if this is right) so I'd imagine it'd survive for a little while out of the water, but I don't think it'd live for too long out of the tank as it'd probably dry-out.
I'd ask for a second opinion but I guess it gives you a little more to google on?