The "sail" is actually an additional anal fin, a naturally occurring mutation which was then further developed by fish-breeders to become a regular trait.
Other common names for this mutant are "Double-finned Knifefish" and "Dimetrodon Ghost Knifefish."
@Chlidonias a study was done and it showed that it was not a re-expression of a "true and proper" dorsal fin, but somehow an anal fin that appears on the "wrong side" of the animal.
