This is a mob found in all of the current hot zones. The level is appropriate for the zone it is in. The loot is typical of any zonewide drops, and an aug progressing in power again based on the zone level.
There is no image of this mob because it masks itself as a normal mob (any mob you pull in the zone could be one) and once it is "found out" it takes the form of its target for the rest of the fight.
How rare do the unmasked changelings seem to be? I was able to get them to pop in Sebilis & CT quite easily, but haven't had any luck here so far. Oh well, at least I'm getting some xps for it.
Alachmed Al'Bedouin Shaman of Noble Blade The Rathe
It would appear that they were frequent spawns at one time, but they have become much rarer now. In an hour period we saw 4 spawn. A week later, we just hunted at the zone in and we got ZERO to spawn in 2 hours.