The Collaborative International Dictionary of English v.0.48:

Redmouth \Red"mouth`\ (-mouth`), n. (Zool.)
   Any one of several species of marine food fishes of the genus
   Diabasis, or Haemulon, of the Southern United States,
   having the inside of the mouth bright red. Called also
   flannelmouth, and grunt.
   [1913 Webster]