The Collaborative International Dictionary of English v.0.48:

Bur fish \Bur" fish`\ (Zool.)
   A spinose, plectognath fish of the Allantic coast of the
   United States (esp. Chilo mycterus geometricus) having the
   power of distending its body with water or air, so as to
   resemble a chestnut bur; -- called also ball fish, balloon
   fish, and swellfish.
   [1913 Webster]