An extra-toes cat isn't a breed. It's any cat born with polydactyly, a harmless genetic trait that adds toes to one or more paws. Many have six or seven toes on a front paw, sometimes forming a mitten-like thumb. The trait is dominant, and it's long been common along the Atlantic coast, where ship cats were prized as mousers.
The Hemingway nickname comes from Ernest Hemingway's six-toed cat, Snow White. Most extra-toed cats are domestic shorthairs or longhairs, so size, coat and personality vary widely, with weights from about 6 to 15 pounds. A cat's age and history say far more about its temperament than its toes do. The main care difference is claw trimming, since extra claws can curl into the paw pads.