The Collaborative International Dictionary of English v.0.48:

Sundog \Sun"dog`\, n. (Meteorol.)
   1. A luminous spot occasionally seen a few degrees from the
      sun, supposed to be formed by the intersection of two or
      more halos, or in a manner similar to that of halos.
      [1913 Webster]

   2. A fragmentary rainbow; a small rainbow near the horizon;
      -- called also dog and weathergaw.
      [Webster 1913 Suppl.]