On Dragons: Rock Imp

I was exploring the veritable warren of caves surrounding the city of Arrix this afternoon when, in a small dark cave, water burbling nearby, I saw a small rock stand up in the firelight and I say! go for my shoe buckle!
The peculiar little beastie had a little beak it snapped at me and chirruped through while I was trying to shake it off. Finally I desisted and cut off my shoe buckle which it then tried to jam into the gaps of its rocky shell. 

- From the journal of the Natural Thaumaturge Aldéric d'Folquet

In planning a previous campaign in my homebrew world, Tiroeth (Tuh-row-eth), my characters had to, well, escape from a city being invaded by the elemental planes, more here. They had to go through the cave system outside the city, and this was just something I created on the spot when they were exploring a cavern. For the next couple of sessions it was a fun little creature to roleplay.

I ran it like a small fey who lives in cave systems and basically pilfers any shiny objects off the party. My groups PC managed to manipulate it by tying a rope to their shoe buckle and giving it to the Imp. It was perfectly content to go along with the party as long as it had access to the rope. Later on it began stealing coins and needles and jamming them in its rocky shell, very chuffed with itself.

One could use it in many ways; it could be the reason why trinkets are disappearing in a castle, or a way to find a treasure in a warren of tunnels (the Imps would be bedazzled in coins and jewels), or a way to give your group a quest, by having it appear with a dangerous - and very shiny - object.