Edited! New and improved. (I think)
The Silent Death. Long rumored to be the ninja's companion, these almost mythical beasts serve a large purpose in the ninja hierarchy. Their snouts are tipped with poison, their wings suited more for stealth and a cloak of sorts rather than flying, a long prehensile tail allows this creature to go where even the ninja fear to tread. All fear the ninja, but the silent death is a being made up of nightmares, and the ninja use this to their advantage.
Each ninja when born is given an opportunity to tame the silent death, sometimes resulting in a lot of dead baby ninjas. But when it works out, when the cads are in the human's favor, he will have a companion, and partner for life. It is unsure where exactly the Silent Death came from. Some say the spawn of old demons and dragons of long ago. Others say that when the mighty Mt Fuji was pushed up through the earth, these beasts emerged from the resulting volcano, born to punish those that deserve to die.