Green’s Dictionary of Slang

an important person, esp. in a business context.

1848		Bartlett Dict. Americanisms 42: In some parts of the country, the principal man of a place or of an undertaking is called the big dog with the brass collar, as opposed to the little curs not thought worthy of a collar.
1858		Wheeling Dly Intelligencer (VA) 8 Oct. 2/1: Monroe, ‘the big dog with the brass collar’ in the cause of the faithful, the stay and hope of Sprigg.
1868		Tarboro’ Southerner (NC) 28 May 4/2: He was the national big dog with a brass collar.
1879		Elk Co. Advocate (Ridgway, PA) 25 Sept. 3/4: The big dog with the brass collar came down [...] to see the small man with the large bald spot.
1884		Fort Worth Daily Gaz. (TX) 7 Aug. 8/4: They say, ‘I am a big dog with a brass collar on; I’ll do as I d—d please’.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1898		Progress (Shreveport, LA) 28 May 4/1: You are all right as long as you are willing to follow the big dog with the brass collar.
1901		Eve. Star (Wash., DC) 13 July 9/1: Bill Clarke is now the big dog with the brass collar among the Senators.