The Fur-Bearing Trout (or Furry Trout) is a fictional creature native to the northern regions of North America, particularly Canada, Montana, Wyoming, Colorado, and the Great Lake. The basic claim (or tall tale) is that the waters of lakes and rivers in the area are so cold that a species of trout has evolved which grows a thick coat of fur to maintain its body heat. Another theory says that it is due to four jugs of hair tonic being spilled into the Arkansas River.
In reality, a possible source may have been a simple misunderstanding. A 17th-century Scottish immigrant's letter to his relatives referring "furried animals and fish" being plentiful in the New World, followed by a request to procure a specimen of these "furried fish" to which the mischievous Scotsman readily complied by making one up, is often cited. In fact, the "cotton mold" Saprolegnia will sometimes infect fish, causing tufts of fur-like growth to appear on the body. A heavy infection will result in the death of the fish, and as the fungus continues to grow afterwards, dead fish that are largely covered in the white "fur" can occasionally be found washed ashore.
The hoax can be unequivocally documented to go back to at least the 1930s. For example, the following is an excerpt from an article in the Pueblo Chieftain dating back to November 15, 1938:
Stuffed and mounted specimens of these fish can be found in a number of museums of curiosities. These are made-up; the Saprolegnia"fur" cannot be preserved by taxidermy, and heavily overgrown fish are usually found only after they have already started to decay.