fixo wrote:

Would anyone have any idea what this thing is? What's it's name? Is it
dangerous? How can it get there? What about that tail?

Looks like a damselfly/dragonfly larvae to me.
http://images.google.com/images?q=da...=Search+Images
http://images.google.com/images?q=dr...=Search+Images

IIRC they are carnivorous, and may be an issue with guppy fry.

It probably got there through an adult either laying an egg in your
tank, or by laying an egg on something that got put in your tank.