The mammal with a black-tipped tail that turns white in winter is the stoat. This small mustelid, Mustela erminea, is also called the Eurasian ermine.
In northern parts of its range, the stoat grows a dense white winter coat that provides camouflage against snow. The tail tip stays black, creating one of its most reliable identifying marks. In summer, the upper body becomes brown while the underside remains pale. Seasonal moulting is triggered largely by changing day length, or photoperiod.
The winter coat is also why “ermine” became associated with luxury and royalty. Medieval and later ceremonial robes often used stoat pelts, and the black tail tips inspired the ermine pattern in heraldry. The animal itself is still called a stoat; “ermine” commonly refers to its white winter fur.
A common mistake is assuming every stoat turns white. Southern populations, including those in Ireland and some other mild regions, may remain brown throughout the year. The arctic fox also changes color seasonally, but it lacks the stoat’s distinctive black tail tip.