One of the oddest of cryptids is an octopus haunting the lakes of Oklahoma, a landlocked state. The reason this is odd is that although freshwater mollusks are common, Cephalopoda (the class consisting of nautiloids, ammonites, belemnites, octopus, squid and cuttlefish) is exclusively marine. The chemistry between fresh and salt water has so little compatibility that few animals manage to cross between both (among those few being salmon, bull sharks, saltwater crocodiles and ancient ambulocetids).
For a cephalopod to transition from salt to brackish/fresh requires certain changes in its physiology. Could such a change be possible, or is a freshwater octopus as likely as an elephant-sized terrasquid?