This is one of my favorite cryptozoological mysteries, although it needs to be explained that correct identification is as important as the discovery of "unknown" animals.
Willy Ley in his books has mentioned Giant Salamanders of the American West, known to early settlers, but extinct by the time that scientific minds had moved to that region.
He suggests that there are many references to giant salamanders of the Rocky Mountains area written by dubious first-hand settler accounts. I want to find some of these, some day.
There are species of documented very large salamanders in Asia. It is not without precedent that they could have had a North American cousin.
On the other hand, misidentification and hearsay and legend and exotic nonnative species are also very real possibilities that need to be examined. Further, some instances may have been legitimate while others not.
ON THE TRACK (of unknown animals) Episode 76
57 minutes ago