The Coral-Backed Devil-Ape
This tropical creature is actually a product of a symbiosis; a union between two different species of animal which have evolved together to benefit from one another's strengths. In this case, a particularly resilient and adaptable species of corallium and an especially hearty breed of orangutan whose ancestors had begun to move into the shallow waters of the ocean.
Through chance, some coral fragments found themselves lodged in the pores of the sea-going apes, and through good fortune, found the nutrients within the simians' flesh that it needed to survive, even during the apes' forays on the land. As the tiny reefs began to spread across the tough hide of the ape, it formed a crude but resilient body armour for the creature, with nearly-razor-sharp ridges and a stone-like consistency.
The apes, for their part, benefited from the protection yielded by this phenomenon enough that, as centuries passed, their physiologies adapted to accommodate it all the better. Nitrogen-rich tissues which contained no essential organs or tissues formed around the bone structures, giving the coral harmless areas to root to, and the diet and habits of the ape species adapted to include the chemicals which the coral needed to thrive.
Finally, the creature now known as the Coral-Backed Devil Ape emerged, bearing the distinctive horn-like formations atop its head from which its name is derived, and a wide, fatty body ideal for movement both in the water, and on the land, where the excess weight of the coral would be mitigated by the ape's low centre of gravity.
Young members of this race frequently spend up to eighty per cent of their time immersed in the water, while their mothers tend to their growing carapace. Older ones have been known to sprout so large a coral reef atop their flesh as to be scarcely identifiable as primates at all, save for their loping gaits and savage whooping calls.