The mantagon (a mantaray dragon)

On the top of a mountain in the middle of the ocean lurks a creature that guards a cave full of treasure.
What it does: it guards it's treasure and occasionally glides down to the ocean floor to find food
Attacks anything that gets near it's cave
It can sometimes lay eggs

What it drops mantagon scales could be used for armor mantagon meat maybe a little better than beef
Mantagon membrane used to make decorative sail
Mantagon skull just a trophy of sorts