Vixen is a cute brown dog with fluffy ears. She's roughly the size of a small Golden Retriever.
Which is big enough to pull a little cart.
Her "mom" is a member of the Society for Creative Anachronism, and in the course of researching medieval dog carts so as to be culturally accurate, she turned up a treasure trove of material. Did you know that the French Canadians have over two hundred years of working dogs pulling dog carts, but that Britain outlawed all vehicles pulled by dogs in 1855? And that dog carts were a popular child's toy, similar to owning a bicycle, for hundreds of years?
More information, and lots of great illustrations, here.