The Goblin Thief is an enemy that spawns during the Goblin Army event. It follows the typical Fighter AI but moves at a higher speed than similar monsters. Approximately one out of every four goblins will be a Thief (23.7% chance to spawn).
Goblin Thieves are between the Goblin Peon and the Goblin Warrior in terms of difficulty. They can jump quite high, and can open doors faster than other goblins.
Despite being called thieves, they never steal anything (except for dropped money in Expert mode). This is referenced by one of the Goblin Tinkerer's quotes.
The Bestiary entry for the Goblin Thief: "Quick strike units of the Goblin Army, these pesky foes specialize in nimble movement and close-quarters combat."