Green’s Dictionary of Slang

a bus horse.

1894		Daily Tel. 10 Nov. 10/5: [For sale] By order of the Star Omnibus Company, six useful busers .
1901		Morning Post 9 Apr. 8/1: Active short-legged bussers. All warranted quiet in harness .
1902		J. Milne Epistles of Atkins 168: Then he went on slowly, all of which showed us that he was one of the old London bussers.