I had never met a Briard dog until just now in the waiting room at the vet's office. Apparently Thomas Jefferson had them. They are French herding dogs.

 The dog was huge and reminded me of the BEAST of MONSIEUR RACINE, a children's book by Tomi Ungerer. I told the woman about the book and she was out the door to get a copy for her 5 year old grand daughter.

"The Beast of Monsieur Racine" by Tomi Ungerer, published in 1971. 

In this story, a retired tax collector named Monsieur Racine discovers a strange-looking creature eating the award-winning pears in his garden. The creature is described as a large, lumpy, weirdly shaped beast, which the community treats as a scientific wonder. 

However, the twist is that the "beast" is actually just two children from next door hiding under a disguise of blankets, rags, and skins. The secret is revealed in a dramatic scene at the Academy of Sciences in Paris when the children break into giggles and come apart from their disguise.