It is essentially a stingrays version of a Cleft Palate.
"Another was born as a Batman ray. The pectoral fins
had failed to fuse together in front of its head in early
development. Its snout was free, and its pectoral fins
projected forward on each side. I was worried that without
a complete disk the ray might not be able to create enough
suction to pull worms out of the gravel. To my surprise,
the ray did fantastic, eating healthily, growing rapidly,
and developing a nice color pattern. I feel that the deformed morphology of the batman ray is due to an error in the process of development. I have a hunch that it is due to some vitamin deficiency, kind of like a cleft lip in humans. I have spoken with J.Langhammer about this and he feels the same way. I suspect that genetic variation in susceptibility to the deformity is likely to be very low, and therefore not breedable. There is no known genetic component to cleft lips in humans, and batman morphology in stingrays almost certainly has nothing to do with genetics."
-Ronald Oldfield, University of Michigan