Unlisted hunts?

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.

I've run into a few as well. Seems a couple of them have regular spawn spots. Got a bat, a bee, and a dragon so far. Missed out on a group of dragons in N Shroud that ended as I came up.