Spiderling
Small enough to sit on a thumbnail, and already spinning for something.
A pet. It is kept on your profile rather than in your bag, so it cannot be sold, banked or lost, and it can be found again. Every find is told in the world's chat.
What drops it
The Old Spinner
1 in 3,000
Rolled on its own on every kill, rather than from the loot table, so it takes nothing away from anything else that falls off it.