A faithful greyhound
In 1400 a French knight, Chevalier Masquer, killed his erstwhile friend Aubrey de Montdidier, and in secret buried his body. But the dead man's faithful greyhound, called Verbaux, led one of his friends to the grave and scratched the ground to expose the body.
Then, whenever the dog saw Maquer he attacked him, although his temperament was otherwise good.
When the king learnt of these events, he ordered a trial by combat to allow God to decide the situation.
Beside Notre Dame, Maquer was buried up to the waist and armed with a stick and shield (the same arrangements were also used for trials between men and women). The dog was let loose and seized him by the throat. The knight screamed for mercy and promised to confess his crime. The dog, presumably, was dragged off - or else super-intelligently let go - and the knight was later hanged.
No record survives of the dog's fate, but one hopes he had the best spot by the fire and a nice meal every night.
(From Duel: A True Story of Death and Honour, James Landale, Canongate, Edinburgh, 2005)
There's a longer version of the story here.
I find it hard to imagine my faithful greyhound doing likewise - as I gather is typical of modern greyhounds he's definitely on the timid side; still you never know, he might be more of a guard dog than he looks.
Since I'm on (possibly shaggy) dog stories, a holiday ahhh tale - Keela, a 18-month-old spaniel and police sniffer dog earns more per day than the chief constable who employs her.
Her sense of smell, so keen that she can sniff traces of blood on weapons that have been scrubbed after attacks, has her so much in demand by forces up and down the country that she is hired out at £530 a day, plus expenses.