The Monster Manual states that Dragon Turtles are “among the most fearsome creatures of the oceans. As large and voracious as the oldest of its land-based dragon kin, a dragon turtle strikes with its deadly jaws, steaming breath, and crushing tail.”
The Monster Manual describes jinni as “rare elemental creatures out of stories and legend. Only a few can be found on the Material Plane. The rest reside on the Elemental Planes, where they rule from lavish palaces.”
The Monster Manual describes Displacer Beasts as a “monstrous predator that takes its name from its ability to displace light so that it appears to be several feet away from its actual location. A displacer beast resembles a sleek great cat covered in blue-black fur. The beast’s eyes glow with an awful malevolence that persists even in death.”
The Monster Manual describes Kuo-toa as “fishlike humanoids that once inhabited the shores and islands of the surface world. Long ago humans and their ilk drove the kuo-toa underground, where they dwell in everlasting night. Kuo-toa can no longer abide daylight."
Beholders are described in the Monster Manual as “the scourge of sentient creatures across countless worlds."
Owlbears are described in the Monster Manual as “a monstrous cross between giant owl and bear, an owlbear’s reputation for ferocity and aggression makes it one of the most feared predators of the wild.”