The scourge of the waterways of Skull Island is the swarming killer-eel. Both appropriately and inappropriately named, the bug-eyed vertebrates are lethal in numbers. When attacking, their razor-sharp teeth shred skin and flesh to ribbons in no time.
Not true eels (or even bony fish at all), but a unique species of hyperoartian related to lampreys, killer-eels stake claim to sections of river they patrol and guard jealously. Groups of up to 100 hunt together. Injured or sick prey (typical fish or reptiles) are the preferred variety. Their tactics involve overwhelming a prey animal with multiple attacks, each member of the group surging in to rasp bite-sized chunks of flesh out of the prey. They will even attack other predators, like Udusaurs.
Slow prey is the mainstay of their diet, as the killer-eels cannot sustain high-energy activity for long periods without resting. For the majority of the time the groups swarm slowly, expending as little energy as possible, until one killer-eel will come upon a potential food source. Then, spurred into activity by the scent of blood in the water, the entire group will attack, overwhelming the prey and often stripping it to the bone with their many-toothed, circular mouths within minutes.
Killer-eel eggs are laid in sticky masses in the water weeds, but suffer dreadful attention, being food to many ray-finned fish and invertebrate species. Coupled with short lifespans, this helps keep their numbers in check. The species is also very vulnerable to changes in water temperature or acidity. Even small changes, if too sudden, can have disastrous consequences for a group.