"the steed of apocalyptic misery"

Four Horsemen of Apocalypse, by Viktor Vasnetsov. Painted in 1887
Public DomainFour Horsemen of Apocalypse, by Viktor Vasnetsov. Painted in 1887

Steed is another word for horse and apocalyptic refers to the end of the world.

The metaphor invites readers to compare the pathetic horse with the four powerful horses of the apocalypse, whose riders Famine, War, Pestilence and Death the Book of Revelation predicts will descend upon the Earth in advance of the Last Judgement.