I have had three of these things appear in the last couple of weeks playing in South Karana. All of them have came from fighting Aviaks in their village. When the Aviaks I am fighting get down to low HPs, these Scouts replace the Aviak. Level does not seem to matter. It is what ever level the Scout is replacing during battle. If you are fighting a level 20 Darter, it appears as a level 20 Scout. If you are fighting a level 24 Advocet, it appears as a level 24 Scout. No one has ever answered the question what these NPCs are for in general chat. I would assume that most mobs in the zone have a chance to spawn one of these. I also assume from their name and the way they replace the mob in battle that they are some kind doppelganger or shapeshifter.
They wear illusions (rogue-like, and why called scouts) that make them become truly visible at low health. They were part of the set of Hotzones ( https://everquest.allakhazam.com/wiki/EQ:Hotzones ) that came out right before Underfoot expansion which is dominated by Cliknars to help promote the release. These mobs drop the hot zone augs for that round. There have been several hotzones sets that included illusory / shape-shifting mobs. One of the worst / most annoying was the mobs for Skal's Walking Nightmare hotzone quest, that upon a mobs death randomly a Skal's Walking Nightmare would respawn at equal to you level, and general difficult to kill. This made soloing without knowing this a bit more dangerous in those zones.
Anyways, that's the jist of it. Seems like the first time they tried this was with Unmasked Changelings in Dec. 5, 2006 hotzones. Possibly earlier from non-hotzone events too, as they used changelings in alot of events over the years.