Magmonix

Magmonix (or Magmonixes) are extremely large, flaming, Treeback-like creatures found in certain regions with lava flows, such as Incline and Mires. Like Treebacks, they are slow to move, turn around and attack. However, they have tremendous amounts of health and because of this are quite difficult to kill. This is possibly due to a bug where a large number of Magmonix (on Mires in particular) are spawned as elites, making them far, far harder to kill than usual.
Apparently their code is almost identical to that of the Treeback, as they use some of the same soundeffects and the same attacks, despite model and location differences.
Magmonix apparently have an extensive and heavy (exo?)skeleton, perhaps the cause for their lack of agility. Magmonix remains can be seen around lava pits (the preferred environment of the Magmonix) on Mires and Incline as large white rock-like features.
They are found in packs, but are not aggressive. Attacking one will not prompt other nearby Magmonix to attack you.