Pistris, Pistrix, Pristis
or Pristrix. The sea-monster sent to devour Andromeda. In ancient art it is represented with a dragon's head, the neck and head of a beast, fins for the forelegs and the body and tail of a fish. In Christian art the pistris was usually employed to represent the whale which swallowed Jonah. (Aratus: commentaries.) Aratus died A.D. 213.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894