The Collaborative International Dictionary of English v.0.48:

Porbeagle \Por"bea`gle\, n. (Zool.)
   A species of shark (Lamna cornubica), about eight feet
   long, having a pointed nose and a crescent-shaped tail; --
   called also mackerel shark. [Written also probeagle.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

porbeagle
    n 1: voracious pointed-nose shark of northern Atlantic and
         Pacific [syn: porbeagle, Lamna nasus]