Green’s Dictionary of Slang

(US) anything or anyone that goes at a great pace.

1861		N.E. Police Gaz. (Boston, MA) 5 Oct. 7/1: Abby, formerly known as the pacer, is still in the land of the living.
1868		Sweetwater Forerunner (TN) 27 Aug. 1/5: In the first heat the pacer led until halfway up the back stretch.
1883		G.W. Peck Peck’s Bad Boy and His Pa (1893) 255: Now, the idea of that man jerking an old pacer. It don’t make any difference if the pacer was a hundred years old, he would pace if he was jerked on.
1887		G. Devol Forty Years a Gambler 222: I did not allow any one to have a faster horse than myself, and generally drove a pacer.
1893		(ref. to 1883) Omaha Dly Bee (NE) 29 Oct. 2/3: In 1883, for the first time, did a horse travel in 2.10, it was old Johnston, the pacer.
1903		Lewiston Eve. teller (ID) 19 Nov. 6/1: The trotting Horse as Against the Pacer [...] the pacer unquestionably holds a slight advantage.
1912		Day Book (Chicago) 4 June 12/1: [He] wanted him as a mate to a pacer he owned. The discovery of the pony’s speed was accidental.
1920		Dly Gate City (Leokuk, IA) 20 Nov. 6/1: In some hard races the rugged little pacer had [...] showed his heels to all of them.