Common Name: Diplocaulus
Species: Diplocaulus Natatorinutrix
Presiding almost solely within island's swamps, Diplocaulus Natatorinutrix is a small amphibian that primarily eats minor fish. It rounds out what I consider the middle-bottom of the ecosystem, feeding on the tinier non-insect creatures of the island while itself being a common snack for the larger carnivores.
Because so many creatures prey on it, Diplocaulus has become very skittish and often flees at the first sign of trouble. It uses its amphibious nature to escape into whichever environment its predator isn't native to. Diplocaulus unique capability to retain vast quantities of oxygen allows to effectively remain submerged for hours at a time, usually outlasting even other amphibious creatures that might otherwise prey upon it.
There are only a few uses for tamed Diplocaulus. It is primarily used for the (rather disgusting) practice of employing Diplocaulus as an oxygen bag. Diplocaulus stores air in the bladders of its head, and divers can suck from these bladders to take deep breaths while submerged, supporting long-term underwater exploration without the use of external gear.