The Collaborative International Dictionary of English v.0.48:

Mulley \Mul"ley\, Moolley \Mool"ley\, n. [CF. Gael. maolag a
   hornless cow, maol bald, hornless, blunt.]
   1. A mulley or polled animal. [U. S.]
      [1913 Webster]

   2. A cow. [Prov. Eng.; U.S., a child's word.]
      [1913 Webster]

            Leave milking and dry up old mulley, thy cow.
                                                  --Tusser.
      [1913 Webster] Mulley

The Collaborative International Dictionary of English v.0.48:

Mulley \Mul"ley\, Moolley \Mool"ley\, a.
   Destitute of horns, although belonging to a species of
   animals most of which have horns; hornless; polled; as,
   mulley cattle; a mulley (or moolley) cow. [U. S.] [Written
   also muley.]
   [1913 Webster] mulligan