The Collaborative International Dictionary of English v.0.48:

bull-necked \bull"-necked`\, a.
   Having a short, thick and muscular neck like that of a bull.
   --Sir W. Scott. [Narrower terms: mesomorphic]
   [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

mesomorphic
    adj 1: having a robust muscular body-build characterized by
           predominance of structures (bone and muscle and
           connective tissue) developed from the embryonic
           mesodermal layer [syn: mesomorphic, muscular] [ant:
           ectomorphic, endomorphic, pyknic]