My dog is polydactyly; she has extra toes.
Wow, I've never heard of that before!  How many extra toes does she have?
Three.  It's unusual with dogs, though.  Cats along the East Coast of North America are often polydactyl.
Interesting!  Are there other animals with this phenomenon?
Yep.  It's a congenital physical anomaly that can also happen in humans!  
Really?  Awesome!  Are there certain breeds of dogs that have this, or can it happen with any breed?
Mongrel
A mongrel, mixed-breed dog or mutt is a dog that does not belong to one officially recognized breed and is not the result of intentional breeding. Estimates place their numbers at 150 million animals worldwide. Although the term "mixed-breed dog" is preferred by some, many mongrels have no known purebred ancestors. Furthermore, crossbreed dogs, while literally a mix of breeds, differ from mongrels in being intentionally bred. Although mongrels have at times been considered somehow lesser than intentionally bred dogs, they are
Polydactyl cat
A polydactyl cat is a cat with a congenital physical anomaly called polydactyly (or polydactylism, also known as hyperdactyly), which causes the cat to be born with more than the usual number of toes on one or more of its paws. Cats with this genetically inherited trait are most commonly found along the East Coast of North America (in the United States and Canada) and in South West England and Wales.
Occurrence.
Polydactyly is a congenital abnormality that can be inherited as an autosomal dominant fashion