Treasure Goblin

A trinket here, a bauble there. A curio from the top shelf. A penny out of a loafer. There's always something to collect when you're somewhere new. The same place day after day gets boring and old. You've seen the best shinies and smoothest of stones. Plus, you've probably already collected the best there is to offer. Sometimes it doesn't belong to you. Almost always it doesn't belong to you; all the better to keep moving. Running, running, hiding, hiding, this is the life of a treasure goblin.
The harmless boogeyman that goes bump in the night, the treasure goblin is driven by the compulsive need to collect valuables and escape. There is something innate in these creatures that they are able to locate and procure the area's most coveted valuables. They don't do research, they don't plan ahead. They simply show up with their satchel, whisk the item away, and move on down the road, giggling as they do.
What the treasure goblin lacks in physical stature and potential for violence, it makes up for in nimbleness and being generally described as a 'slippery little bastard.' It is near impossible to catch these burglars in conventional traps as they spend every waking hour that is NOT spent collecting, evading capture.
They have built themselves such a reputation that bands of people will set off on little quests just to hunt them down. Sometimes not out of spite or revenge at all. Sure that definitely happens when priceless heirlooms go missing and the little bugger was spotted leaving town. Treasure hunters will be treasure hunters, whether it's in a cave of wonder, a shipwreck off a dangerous coast, or a little cackling greenskin hauling around a bag of loot. They know it's loot. These creatures have neither the strength nor desire to carry garbage or junk around.
If an adventurer is ever fortunate enough to encounter such a creature, one might be faced with a unique opportunity to befall riches. The catch is, these creatures know how to live on the edge of danger. It is almost impossible to expect one will randomly happen upon a wandering treasure goblin on a peaceful night of pleasant weather. No. These critters hug the edge of the dragon's lair, scurry around during your high stakes heist, or take the opportunity to make their move when the team has infiltrated the governor's mansion to plant incriminating evidence. Their safety relies on your peril.