“From Tartarus’ heart I stab at thee”
 
Mirrorca are rare magical beings who live high in the sky and feed on smaller flyings animals which include pegasus (although they are usually too fast to get caught). Their skin possess the incredible ability to reflect any spell thrown at them.
 
In ancient Equestria, they were hunted for their skin which was used to make light and magic resistant armors, but due to their rarity and the fact the skin tend to lose its reflective properties over time, the species is now nearly extinct and even a small piece of skin is worth thousands of bits on the black market.
 
Although their thick skin makes them impervious to magic and very resistant to physical weapons, their eyes and mouth are not protected and can be targeted to avoid damaging the skin too much, but doing so will not kill the mirrorca immediatly, exposing the hunter to their anger.  
Captain Celaeno experimented this first hand when she and her crew angered a mirrorca by piercing one of its eye, they all managed to escape safely but she left a part of her behind.