Skum

Skum is a hostile level 20 elite thunder lizard found in the Wailing Caverns.

Background
A great thunder lizard captured from the vast plains of the Barrens, Skum has quickly become Lord Pythas' favorite pet. Strengthened by the corruption of Naralex's nightmare, Skum is a vicious beast, and should not be underestimated.

Dungeon Journal
''Skum first journeyed to the Wailing Caverns for relief from the Barrens' hostile environment. Like many of the caves' other inhabitants, he quickly succumbed to corruption. His veins now pulse with dark energies that have transformed him into a mighty and ruthless beast beyond redemption.''

Summary
Skum is just an elite thunder lizard. Kill the 2 druids in front of his pen, and then pull him. When he is at very low health, he will run away in fear, just finish him off when this happens.