I was looking around on Pokemon Showdown when suddenly I came across this beig named:Stratagem'.
It looks like it's a legendary Pokemon that got cut from B/W(2).
If someone knows more about this 'thing',please tell me.
It's not a real Pokemon. It's a Pokemon from Smogon's Create-a-Pokemon project, where they create Pokemon and use them in a customised metagame called CAP. Stratagem is one of many Pokemon they have created for it. You cannot use these fan-made Pokemon in standard matches, only in the special CAP game. So unless you're interested in CAP, just ignore it.
More info on the CAP project here.