As rediscovered by Salvor Hardin, in Foundation by Isaac Asimov:
“A horse having a wolf as a powerful and dangerous enemy lived in constant fear of his life. Being driven to desperation, it occurred to him to seek a strong ally. Whereupon he approached a man, and offered an alliance, pointing out that the wolf was likewise an enemy of the man. The man accepted the partnership at once and offered to kill the wolf immediately, if his new partner would only co-operate by placing his greater speed at the man’s disposal. The horse was willing, and allowed the man to place bridle and saddle upon him. The man mounted, hunted down the wolf, and killed him.
“The horse, joyful and relieved, thanked the man, and said: ‘Now that our enemy is dead, remove your bridle and saddle and restore my freedom.’
“Whereupon the man laughed loudly and replied, ‘The hell you say. Giddy-ap, Dobbin,’ and applied the spurs with a will.”
Further reading from the ACLU (via Glenn Greenwald).