This is one of the most famous shaggy dogs. Literally.

In the days of yore, a knight was on his way to do something terribly
important, riding his horse into the ground to get to his destination
as fast as possible.

After being ridden too hard for too long, his horse became lame, and
seeing a small town ahead he headed straight for the stables there.

“I must have a horse!” he cried “The life of the King depends upon it!”

The stable keeper shook his head. “I have no horses,” he said. “They
have all been taken in the service of your King.”

“You must have something – a pony, a donkey, a mule, anything at
all?” the knight asked.

“Nothing… unless… no, I couldn’t”

The knight’s eyes lit up. “Tell me!”

The stable keeper leads the knight into the stable.

Inside is a dog, but no ordinary dog. This dog is a giant, almost as
large as the horse the knight was riding. But it is also the
filthiest, shaggiest, smelliest, mangiest dog that the knight has
ever seen.

Swallowing, the knight said “I’ll take it. Where is the saddle?”

The stable keeper walked over to a saddle near the dog and started
gasping for breath, holding the walls to keep himself upright.

“I can’t do it.” he told the knight.

“You must give me the dog!” cried the knight. “Why can’t you?”

The stable keeper said “I just couldn’t send a knight out on a dog
like this.”