The Bodhisatta was once born as a bull as black as jet. While still a young calf he was given by his owners to an old woman, who reared him like a son; hence his name ("Grannie's Blackie"). The village lads used to ride on him for play. Once he saw a merchant trying to get his five hundred carts across a ford; the merchant's bulls were not strong enough for the task, and seeing the Bodhisatta the merchant tried to make use of his services. The Bodhisatta agreed only after he had been promised a thousand. The task completed, the merchant tried to cheat him, paying only half the promised amount.
But the bull would not let him go till all the money had been paid. The earnings so obtained he took to the old woman, who was greatly pleased. J.i.194-6.