Streaked Devourer

It seems highly likely that the streaked devourers are a more adolescent version of the usually more numerous cave devourers. It's debatable if the factor in this change is really physical age or just the amount or quality of devoured material, but such thoughts become pointless when confronted with a ravenous mass of teeth that want to destroy you. Their power and ferociousness are strengthened over time as is their toughness. It's still driven by eternal hunger and shows no more restraint in following it's primal impulses than his younger self did. Before entering the transformation into a streaked devourer, the creature swells to twice its size and becomes rather slow. In such a phase it's extremely vulnerable and can hardly fight back. The transformation takes place in a couple of weeks in which the devourer retreats to a remote spot. In the end of the transformation the devourer sheds its old skin which had dried up and wrinkled away over time.
Sometimes a devourer that is transitioning into the next phase becomes immobile and over time turns more and more into stone. It is probable that such specimens lack the right minerals to go through the whole transformative process to become a streaked devourer.
In very rare occasions some specimens were encountered in a phase to transition between stages, but such creatures seem to seek out remote places and commit to their transition completely. The streaked devourers are usually loners and highly territorial. Thus infights are common, though rarely deadly. Only through some outer influence they are coerced into cooperation. If part of an army for long enough, their attitude to unwanted company seems to soften up a bit and they learn to cease their aggression even without constant supervision.
The pheromone that streaked devourers mark their territory with is, in case of the 'domestic' counterparts, used in advance packs to mark a path that the main army can follow up later. Gnomish special operation teams reported some success in diverting armies in certain direction due to the use of salvaged pheromone glands.
As streaked devourers become more and more magnetic over time, in some cases movements of greater numbers of streaked devourers could be tracked by using a compass.