A Boar dirtied water a Horse wanted to drink. Going to Man, the Horse agreed to be ridden to the water so Man could kill Boar. Horse never got released.
Be careful how you take revenge; you could end up a slave.
L’Estrange version
A boar happened to be wallowing in the water where a horse was going to drink, and there grew a quarrel upon’t. The horse went presently to a man, to assist him in his revenge. They agreed upon the conditions, and the man immediately arm’d himself, and mounted the horse, who carry’d him to the boare, and had the satisfaction of seeing his enemy kill’d before his face. The horse thank’d the cavalier for his kindness, but as he was just about to take leave, the man say’d he should have further occasion for him, and so order’d him to be ty’d up in the stable. The horse came by this time, to understand, that his liberty was gone, and no help for’t, and that he had pay’d dear for his revenge.
Moral
Let every man take a true measure of himself, what he is able to do, and what not; before he comes to any peremptory resolution how to proceed. He is a madman, that to avoid a present, and a less end, runs blindfolded into a greater; and for the gratifying of a froward humour, makes himself a slave all the days of his life.