Dictionary of Ichthyology  

furfish = a taxidermist's mount of a fish, often a trout, wrapped in white fur and said to be a denizen of Canada's northern waters (or northern U.S. states). Sold to tourists. The original idea could be from fish infested with the fungus Saprolegnia which appears as tufts of "fur". The fungus continues to grow after the fish dies and is washed up on shore and can cover much of the body.

From the Royal Scottish Museum, Edinburgh,

Canadian Museum of Nature