There are several reasons that make it more creditable for me. For one, its not as well known as Bigfoot and is a strange tale to make up, for another the eye witness accounts that go back to the 1800’s describe biological features that the witness didn't understand, but are discernable to those who are familiar with salamander biology. For instance, as mentioned before gills structures are described as antlers. If you were going to make up a story about a giant salamander would you add antlers? Probably not. That convinces me that they saw something. Another sighting described a giant worm like creature with two arms. In this case neither the witness nor the person reporting it seemed to be aware that large neonate salamanders like Nectorus sp. do not develop all their limbs until they are mature in some five to six years, again lending credence to their account. For another, giant salamanders do exist, the Japanese giant salamander, the North American hellbender, and the Congo eel. As for the likely hood that a giant salamander could go unnoticed, I can only ask, have you ever tried to find a mole salamander like a tiger salamander or spotted salamander outside of it breeding season? They are extremely difficult to find, even in areas were they are plentiful. The only time that they are found is when they congregate for breeding purposes. Can you imagine trying to find a rare species of salamander that breeds in underground pools?