It seems that Blake's definition of oxen is likely the direct source of Hardy's choice, although I am still quite interested in oxen in prior mythological canons. (Hardy lived in an era when many ideas from Asian cultures were making their way to England.)
From A Blake Dictionary: The Ideas and Symbols of William Blake by Samuel Foster Damon:
The Ox is a castrated bull, who drags the plow, and is slaughtered for meat. He is docile and patient, although there are "moping terrors" in his brain. "He who the Ox to wrath has mov'd / Shall never be by Woman lov'd"; it would require an excess of cruelty to rouse the wrath of such a gentle creature or his human equivalent.
Source: Google Books, ibid. p.313
Based on Hardy's political and social views, the ox is a much better representation of the common man in an industrial society, as a docile beast of burden whose ultimate fate is as "meat for the machine". Although commoners may not have owned land in pre-Industrial societies, farming, even nominally, for oneself is quite different from factory work. (See Orwell's Animal Farm and Shaw's Mrs. Warren's Profession, which has implications beyond feminism, for similar explication of economic systems as means of controlling human bodies.)
The idea that it requires an "excess of cruelty" to rouse the beast to wrath may be taken as a comment on labor conditions leading to strikes and massacres of workers in Industrial societies up through the early 20th century. This may not be implicit in the poem, but is supportable.
More implicit is the association of this poem with the first World War, during which it was published.