The Thief is an enemy in Gauntlet.
The Thief is a very fast enemy that appears in some levels later on in the game. He does 10 points of damage, and also steals something from you. The item he steals depends on what you have. If you have any upgrade potions, he will go for those first. Otherwise, he might nick your potions, keys, score, or multiplayer bonus multiplier.
The Thief spawns at the beginning of the level, and runs toward you. If you hear the Thief tone, get into a choke-point and turn around to start shooting. He will run right into your shot, and leave behind a treasure bag worth 500 points.
If you get robbed, you can kill the Thief right away to recover what he stole. If he exits safely, you can find your stolen item on the next level. Unfortunately, upgrade potions become regular potions when stolen (although, in the late-game phase, this can actually be helpful to obtain another regular potion to clear an area).