I don't recall coming across the term road agent before I read Cormac McCarthy's The Road. It means a highwayman or robber of stage coaches. And it's got a wonderfully sinister tone - amplified in McCarthy's tale by the fact that his road agents are murderers and cannibals too
Murderous robbers play a big part in the Greek myths. As I put together a campaign with an ancient-world flavour, I've been thinking a bit about these. The best examples are the eccentric murderers encountered by Theseus on the road to Athens: Sciron, Procrustes and the rest.
These robbers get up to all sorts of unsavoury tricks. They stretch people to fit beds - or lop off their limbs to do the same. They kick people from cliffs, or wrestle them to death, or beat them with cudgels, or tear them apart with trees.
They also provide a tremendous template for RPG encounters. The PCs need to get somewhere, but there's an obstacle in their road. They can go round it (creating an opportunity for wilderness or maritime adventures) or they can go through it.
That entails dangers. Here's Plutarch on the robbers on Theseus's road to Athens:
For verily that age produced men who, in work of hand and speed of foot and vigour of body, were extraordinary and indefatigable, but they applied their powers to nothing that was fitting or useful. Nay rather, they exulted in monstrous insolence, and reaped from their strength a harvest of cruelty and bitterness, mastering and forcing and destroying everything that came in their path.
Some of these robbers are demigods or literal monsters, like the cyclopean club-wielder Periphetes. Some of them also guard entrances to the underworld and consort with monsters: a giant sea turtle or a savage giant boar.
While the brief ancient texts can imply that these robbers were solitary individuals, some have families with them, and it's easy to imagine them holding court among thronging followers: as bandit chiefs rather than solitary brigands. That adds further scope to an RPG encounter: approaching the haunts of an infamous bandit might involve ominous portents, surly guards and obsequious underlings. Think of Jabba the Hutt's palace - or Kurtz's lair in Heart of Darkness:
An encounter with a road-agent of the sort that Theseus faces might, in a gaming session, include interactions with all manner of ne'er-do-well, opportunities for audiences with the monstrous bandit-king himself, and the chance to witness a horrific execution - or become an involuntary participant in one ...