There are lots of fish that are suited for surviving in cold temperatures, including goldfish, koi, carp, shubunkins, roach, and other kinds of fish. Fish are cold-blooded, meaning that the ambient temperature around them helps regulate their internal body temperature. This is what allows their internal systems to change and alter with the seasons, such as slowing down their metabolism and resting when things get cold. Which fish are best suited for winters in your area will depend on the severity of your seasons, and how you prepare your pond for the season.
All living beings require oxygen to survive, and even fish in a state of torpoer (a sort of semi-hibernation) need to breathe. Your job, as a pond owner and fish-buddy, is to make sure that your pond is able to maintain gas exchange, especially during the coldest months when the water is frozen over. Keep a small section of of the surface free of ice so that toxic gases (from decaying materials) can escape and oxygen can dissolve into the water. In the relatively small environment of a pond or small lake, maintainging this clear area is critical to the survival of your fish over the winter.
