The Collaborative International Dictionary of English v.0.48:

Bull Moose \Bull Moose\ (U. S. Politics)
   (a) A follower of Theodore Roosevelt in the presidential
       campaign of 1912; -- a sense said to have originated from
       a remark made by Roosevelt on a certain occasion that he
       felt "like a bull moose." [Cant]
   (b) The figure of a bull moose used as the party symbol of
       the Progressive party in the presidential campaign of
       1912. -- Bull Mooser. [Cant]
       [Webster 1913 Suppl.]