Wasn't sure what section of the forum to put this in sorry if its wrong.

But I came across a level 50 mob in one of the shrouds, I was on my way to turn in the tam tara HM quest and it was called The White Joker and was level 50. It seemed odd to me so I killed it and got tomes of soldiery/myth. Are these mobs random rare spawns? It wasn't on my daily hunt or elite hunt marks either.