Ugluk sounds like an Orc name, so I'm guessing one of those in the tower near the Cirith Ungol pass. Could be wrong, though. Bill is the pony. The other three are historical characters who aren't actually in LotR but apparently are mentioned often enough to make the list. Amroth is, if I'm not mistaken, the human who had a thing with some Elven girl (Nimrodel? or am I mixing up couples there?), and the people of the city of Dol Amroth in Gondor - like Prince Imrahil - still have some Elven blood in them. Helm Hammerhand is one of the kings of Rohan, the one who built Helm's Deep. And Arvedui is some ancestor of Aragorn's, the last one of the kings of the North iirc. If I had to guess, I'd have to suppose he gets a fair few mentions when the hobbits are on or near Weathertop. Amazing that he'd make it to the list over e.g. Cirdan, though.