I can tell you right here and now, IT IS HACKED, as you said in your post, Zekrom is one of the shiny-locked Legendaries, and if it is shiny and W/O the pentagon, IT IS HACKED. The best way to check if Pokemon are checked is to use a dichotomous key like I do, and if it is legal, I usually run it by a friend to get a second opinion. I usually give everything BUT legendaries the benefit of the doubt, and I usually suspect legends from the beginning, because they are often hacked. You can also use the memory checker.
Source: Experience in hacking, shinys, and hack checking!
Hope I helped!!