This is one of the two monsters currently dominating the 'Goblin Village'. While having fairly low health and damage in comparison to many other mobs, Goblin thieves make up for it with their tenacity. The goblin thief mob is most known for it's bag of items, which it will hold on it's back. This is presumably why it tends to drop more items upon death in comparison to the normal goblin.
When fighting goblin thieves, never forget that when you kill one, another will very soon follow to avenge it's fallen brethren. Goblins in the 'Goblin Village' are most notable for their ridiculously high spawn rate. This alone is one of the reasons the goblin village is often avoided by low-leveled players, as they would be overwhelmed by sheer number should they make their stay in the 'Goblin Village' for too long.