Green’s Dictionary of Slang

(US) the leader, the boss.

1927		S. Graham N.Y. Nights 30: The Russian he-bear, the bull-moose of those parts, holds his wine-glass over the cendre hair of his picnic companion.
1940		E. O’Neill Iceman Cometh Act I: De Bull Moosers is de on’y reg’lar guys.
1958		McCulloch Woods Words 21: Bull moose—a. A large anything. b. The push, or head man on a logging show [HDAS].