Green’s Dictionary of Slang

a red herring (the fish).

1874		Hotten Sl. Dict. 82: Billingsgate Pheasant, a red herring or bloater.
1879		Punch 77 285/2: No, no, my dear ’enery, a ‘Billingsgate pheasant’ is not a bird; and a ‘two-eyed steak’ has no connection with beef. They both mean the same thing, viz., a bloater.
1883		‘One of the Crowd’ Mysteries of Modern London 113/2: ‘What with fourpence to pay for medsun for the young ’un [...] if it runs to a Billingsgate pheasant for supper it’s about as high as it will run.
1884		Dundee Courier (Scot.) 18 Jan. 5/6: Some folks call it a Billingsgate pheasant.
1885		Cornishman 7 May 6/5: What is that i see — Billingsgate Pheasants! Just give me a couple!
1889		Essex Newsman 5 Nov. n.p.: What are the odds that he didn’t get [...] an appetising rasher or a savoury Billingsgate pheasant for his tea?
1894		Leeds Mercury 8 Sept. 12/1: A red herring or bloater split in halves; otherwise known as a ‘Billingsgate pheasant’.
1898		Belfast News-Letter 11 Apr. 6/5: A herring is hardly recognisable as a ‘Billingsgate pheasant’ or a ‘two-eyed steak’.
a.1909		press cutting in J. Ware Passing Eng. of the Victorian Era 31/1: This paper always comes useful, if it’s only to wrap a Billingsgate pheasant in to take home to the bit of tripe.