peacock horse, n. — Green’s Dictionary of Slang

peacock horse n.

[such horses are bedecked in mourning black, with plumes, and step in a very ceremonial way]

a horse that pulls a hearse.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 187: peacock horse amongst undertakers, is one with a showy tail and mane, which holds its head up well.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1909		J. Ware Passing Eng. of the Victorian Era.