I found this LEGO piece in a second-hand LEGO stock. I have never seen a brick like this, and I can't even find the LEGO write on it.
Is this LEGO brick original, maybe vintage or some kind of special edition or is it just fake?
This is a Minitalia brick - Lego produced different bricks for a while in the seventies for Italy (per Gary Istok's latest information: because the original LEGO was deemed too expensive). Minitalia bricks have weaker plastic (which shows on the studs on your brick) and don't use the Tube system underneath the brick, but an older patent which has the X marks. The gripping power of the X marks is less good, but you will find that the X marks actually allow for half-stud offsets, which you cannot do with the tube bricks :). All this is detailed in Gary's wonderful book/DVD
It's definitely Minitalia and since you're Italian let me suggest a very recent Italian article with the most recent findings about the history and the actual reasons behind the existance of the Minitalia line!
I also recommend Gary's book, it's really great (and he's working on a new edition that will include these recent findings).