Heterocercal is a coordinate term of diphycercal.

As adjectives the difference between diphycercal and heterocercal

is that diphycercal is describing a symmetric tail fin in which the vertebral column extends to the tip while heterocercal is of a fish's tail, such that the vertebral column bends and extends upwards into the upper lobe of the tail, making it larger than the lower lobe; as in sharks, rays, and skates.