The ferret is a domesticated mammal of the type Mustela putorius furo. Ferrets are sexually dimorphic predators with males being substantially larger than females. They typically have brown, black, white, or mixed fur, have an average length of 20 inches (51 cm) including a 5 inch (13 cm) tail, weigh about 1.5–4 pounds (0.7–2 kg), and have a natural lifespan of 7 to 10 years.
Several other small, elongated carnivorous mammals belonging to the family Mustelidae (weasels) also have the word ferret in their common names, including an endangered species, the Black-footed Ferret. The ferret is a very close relative of the polecat, but it is as yet unclear whether it is a domesticated form of the European Polecat, the Steppe Polecat, or some hybrid of the two.