There are four different kind of looters in Chariot - Hoppers, Flyers, Flippers, and Ropers. As you progress through the levels you will eventually come across all four, it's just a matter of time.
Hoppers - First looter you come across. They hop around and are limited by high walls and ledges.
Flyers - Second type of looter. These guys are not limited and can fly through walls and obstacles to steal your loot.
Flippers - Third looter you will see. These guys walk along the ground and will 'flip' you into the air. If you are attached to your chariot with the rope the flip will also disconnect you from it. These guys are very annoying in later levels.
Ropers - The last looter type. These guys will attach onto your chariot with a rope of their own. This makes it rather difficult to pull it along yourself. They also suck in their breath to pull the chariot close, but this seems to not have too much of an effect.
EDIT: The first time you come across the Roper is in level 4-2. When you drop into a lower cave and there's a life bridge blocking you on both sides and ghost rail in the center. You need to drop below the chariot to continue. That is where the first Roper is.