A Twice-Told Tale with a Tail-End

Told by Aesopus, I think—and then by Bierce.

I'll tell you a tale: 

A Lion who had caught a Mouse by the tail was about to kill him, when the Mouse said: If you spare my life, I may do as much for you some day. The Lion laughed to tears and then good-naturedly lifted his paw and let him go. But it happened short afterwards that some hunters set a trap, and the Lion was caught in a net and couldn't move. The Mouse, passing that way and seeing that his benefactor was helpless, gnawed off his tail.

—oOo—