King Arthur sends Sir Lancelot out on an important mission to deliver a message to the king of Spain. It is a long distance, and Lancelot looks in the Kingdom for a good horse to take him there. His own horse is sick, and all he can find is an old mare, but, since he has to leave quickly, he takes the mare.
About 3 days out of the Kingdom, Lancelot realizes his mistake. The horse gets tired and appears to be going lame. He finally makes it to a small village and gets to the Inn. He goes up to the Innkeeper and explains his problem. That is, he needs a good horse so that he can fulfill his mission to deliver the message for the king. The Innkeeper replies that this is only a small village, and most of the horses around are not up to the task. He is welcome to look around, however, and if he can find anything, he is certainly welcome to it.
Lancelot looks around the village, and true as the Innkeeper has said, no good horse is to be found. As Lancelot is about to give up, he comes across a stable boy carting some feed. He asks the stable boy if there is any beast of burden in the village that he can use to fulfill his mission. The stable boy thinks for a minute, and starts to reply no, but then says, go see if Old Mange in the barn can help you.
Lancelot goes over to the barn expecting to find a horse. What he finds is a very large dog: almost as large as a pony. The dog is a mess, however. It is mangy, parts of its fur are falling off, and it is full of fleas. Lancelot is desperate at this point, and he looks it over carefully. It does, however, appear to be strong enough to take him to Spain (which is only 3 days away at this point).
Lancelot goes back to the Innkeeper, and acknowledges that he cannot find a horse in the village that he can use. He says, however that this dog, Old Mange, might be able to take him most (if not all) of the way to his destination. The Innkeeper hears this, stiffens up, and says : Sir. I wouldn’t send a Knight out on a dog like that.