Found within underwater cave systems is a peculiar aquatic reptile known as the Vanuctadon. Standing around the size of your typical humanoid, these cave dwellers are covered in a layer of smooth black scales. Perfect for camouflaging themselves in these dark caves. Despite their size humanoids rarely need to worry about Vanuctadons, as their favored foods are cave fish and insects. In order to catch their preferred prey, these crafty reptiles construct pools made of mud found within the caves. They then scare the cave fish into these small pools to restrict their movements. At that point the Vanuctadons needle-like beak is able to spear the slippery fish before they can escape the pools. These constructed pools are often used by explorers to calculate how many Vanuctadons are within the cave system. However, within these dark cave waters visibility is quite low and the Vanuctadon have especially poor eyesight from generations of living in these dark waters. Luckily Vuncatadons developed a form of echolocation to make up for the lack of visible light. Even when not hunting this echolocation allows them to navigate their lightless world easily. The most striking feature of Vanuctadons are their tall dorsal spines running along the back. These sails function as both a mating display and a form of defense. On top of the sails are sharp spines that can pierce attackers or larger Vanuctadons. To avoid deadly physical battles between rival males, the Vanucdaton size each other up. Which usually involves walking side by side in slow deliberate manners. That way the individuals involved can truly tell who has the larger sail and thus is the stronger opponent. 

Credit to thedesignateddm and dinnkrnwn, for some lore ideas.