- Order: Piscine Wyvern
- Suborder: Fish Feet Wyvern
- Superfamily: Water Wyvern
- Family: Plesioth
Within the Old World Plesioth have been seen in living in Jungle, Old Jungle, Desert, Old Desert, Misty Peaks, and Great Fortress Lake. The aqautic wyverns also inhabit the Deserted Island, and Flooded Forest of the Moga region.
Plesioth is a large ambush hunter. The creature can hunt both fish underwater and prey on the surface coming too close to the waters edge. Its defenses are somewhat poor, and it will try to avoid any kind of confrontation, be it on the surface or underwater. Plesioth are somewhat territorial, actively patrolling its turf until it finds a suitable meal. On land it is most likely high in the food chain, due to being so large many monsters would not attack it. However, due to their limited mobility out of water, they are at risk from attacks from other large, more agile monsters, such as a fire wyvern or Tigrex, so they never venture far from their native rivers or lakes. Plesioths are more dominant underwater than they are on land, since there is less competition, challenges would come mainly from the Leviathans, with Lagiacrus being a major threat, seeing as Plesioths are vulnerable to electric attacks, which the Lagiacrus utilize, but it is unknown what reaction they might pull if interacting with the gigantic Ceadeus. Plesioth have also been known to prey on large serpentine crocodiles that live within the Old Jungle.
Plesioth do not possess gills for breathing underwater, so they have to come up for air every so often to fill their lungs with air. In addition to having lungs these creatures can also breath through their skin like an amphibian or sea snake. Inside their fin's spines, a horrific neurotoxin can be found in them. This toxin can make a victim fall asleep with a single touch or a single bite. The fins of Plesioth allow it to swim fast enough to pass a galloping horse. Covering a Plesioth's body is these shiny black scales used to hold in moisture. A scale from older Plesioth can fend off claw and fang attacks. Plesioth have powerful jaws that can crush the shell of Carapaceon, and other armored species, making preying on those species easier. One of its most well-known features is its ability to breath high-pressurized water at its foes. The water that it uses is swallowed while swimming before Plesioth actually uses it as a weapon.
Some rare individuals in the Old World appear to have more muscle mass to the point were there legs are thick as a Flying Wyverns, have unique wing patterns, more vivid body color, are more aggressive to the point of using a sweeping beam on land and using new techniques to attack intruders, prey and predators including body-slams, jumping over and ahead of its target doing full U-turn mid air then slithering on the target, faking sleep, charging on foot in the same manner as it would to return to the water trampling and stabbing targets and using its sleep inducing liquid in bursts on land splashing it on predators and prey who approach.
Female members of this species are viviparous and like some sharks the first of the offspring to hatch will eat their either unborn or smaller siblings within the womb.
Plesioth will confront attackers from their aquatic home and will even come on land to fight hunters if they are angered enough. They dislike loud noises very much, and will flail about when audibly provoked, as their hearing is extremely sensitive.