Abstain from the Melanurus. This is the sixth symbol in the Protreptics. Melan-uros means the “black-tailed.” Pythagoras told his disciples to abstain from that which has a black tail, in other words, from such pleasures and pursuits as end in sorrow, or bring grief. The Melanuros is a fish of the perch family, sacred to the terrestrial gods.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894