The most frustrating cases in zoology / cryptozoology are those where a single specimen exists, but its provenance cannot be proved, and no one can find another. One such case is a cloak of kiwi feathers from New Zealand in which the feathers are too big for ordinary kiwis: was there, in Maori times, a giant variant? Another such case, where the specimen is not preserved but unquestionably existed, is the Case of the Wayward Salamander.The largest living amphibian is the Asiatic giant salamander Andrias davidianus. Found in China, with a slightly smaller relative in Japan, it may be over 1.5 meters long. The largest known in North America is the hellbender of the Ozarks and Appalachians, which grows to over 60cm.
America's largest known salamander, the hellbender (Wikimedia Commons)In 1951, an article by Stanford University herpetologist George Myers appeared in the scientific journal Copeia. Myers reported he had examined a giant salamander caught in a catfish net in California's Sacramento River. The amphibian looked like an Asian giant salamander to him (he called it a member of the genus Megalobatracus: Andrias has superseded the older name). However, it was dark brown with yellow spots, "quite at variance" with the standard gray or brown of Asian types, and "suggested the possibility of a unique California variation." The specimen (if fully grown) was of modest size for this genus, only 76 cm long. Unfortunately, the fisherman only allowed the specimen to be examined, not kept for further study. What became of the salamander, then living in a bathtub in its owner's apartment, is unknown.
A point here is that the coloration isn’t, in fact, outside the range of Andrias. A Google Images search turns up a brown one with yellow spots, or at least splotches.
Getting back to California, giant salamanders have long been rumored from the Trinity Alps. A deer-hunting attorney claimed he’d seen five such animals in the New River, ranging up to almost three meters long. Biologist Thomas Rodgers, who investigated this story, allowed it was possible that a relict population of giant salamanders still lived in California.
Rodgers also looked at the Sacramento River specimen. The local press related a rather too cute story that this salamander was an escapee named "Benny," from a shipment of exotic pets from "somewhere in China." With the question thus properly muddled, Rodgers led an expedition into the Trinity Alps in 1961 to search for the alleged amphibians. Rodgers' group found only well-known native salamanders under 30cm, and he came away doubting any giant salamanders existed.
Other reports have trickled in since then, but no one has produced another giant salamander. We are left with the usual problems of one-specimen cases. Was the animal an exotic escapee? A last relic of a lost race? A chance specimen of a surviving population? I hate writing "we may never know," but, well...
See: Myers, George S. 1951. "Asiatic Giant Salamander Caught in the Sacramento River and an Exotic Skink Near San Francisco," Copeia, No. 2: Rodgers, Thomas L. 1962. "Report of Giant Salamanders in California," Copeia, No. 3.