I'll say it's very rare! The explanation behind this very rare fish is described on the main plaque below it:

"The great fur-covered trout myth: A furry fish like this was once displayed in the National Museum of Scotland as an authentic species of trout! Believing that the immense depth and icy temperature of Canada's Lake Superior made the fish grow fur to keep warm, the museum stubbornly refused to believe the fish was a hoax.