The Collaborative International Dictionary of English v.0.48:

Howler \Howl"er\, n.
   1. One who howls.
      [1913 Webster]

   2. (Zool.) Any South American monkey of the genus Mycetes.
      Many species are known. They are arboreal in their habits,
      and are noted for the loud, discordant howling in which
      they indulge at night.
      [1913 Webster]

WordNet (r) 3.0 (2006):

howler
    n 1: a joke that seems extremely funny [syn: belly laugh,
         sidesplitter, howler, thigh-slapper, scream, wow,
         riot]
    2: monkey of tropical South American forests having a loud
       howling cry [syn: howler monkey, howler]
    3: a glaring blunder