The facts that started the legend of King Arthur are:
1) Arthur was a man that lived and led in Britain.
2) Arthur never called himself a king.
3) Arthur loved his dog.
Legends of King Arthur speak of Cavall, a mighty hunter and boon companion. Cavall was Arthur's "favourite dog". One of the only facts that can be documented about Arthur is how he wept upon his dog's death.
Guinevere, my dear
You can't hold a light
The canine time
Gives us might
Broke his heart
When they'd part
No matter how quick
Merlin's magic
You would have to concede
Not up to dog's speed
Broke his heart
When they'd part