[symestreem]

But Butterbur HAD encountered the Nazgul. Strider knew this. The implication is that the Rangers prevent this unknown foe from entering Bree. But they do enter Bree, so I don't think Strider is talking about the Nazgul. If you take it in context, Aragorn is talking about what the Dunedain have always done, and they haven't always been guarding against the Nazgul. Earlier in that speech he says "But when dark things come from the houseless hills, or creep from sunless woods, they flee from us." Dark things from houseless hills seems to imply barrow-wights. And in RotK, Butterbur is talking about how things have changed since the Rangers left: "For there's been worse than robbers about. Wolves were howling around the fences last winter. And there's dark shapes in the woods, dreadful things that it makes the blood run cold to think of." Blood run cold- heart freeze? Kind of seems like the same think that Aragorn was talking about. So these "foes" might barrow-wights, wolves, robbers, or unkown dark shapes in the woods. Bree is pretty close to the Old Forest on the map, about 20 miles, which is a day's march.
Perhaps these things were left over from the Witch-king's rule, and crept south for new prey? They might do so especially if their former king was near.