A race of people who, like the N'Tuitif of Howondaland, are pathologically incapable of lying. This is a drawback in a trading race, so the Zoons elect a Tribal Liar to do the negotiating for them. Other people get offended by this and think they should have picked a more acceptable term, like "diplomat" or "politician". The Zoons reply that this is exactly the point.
The Zoons run cargo barges between Ankh-Morpork and the hinterland. They'll also carry passengers, but presumably charge high prices, and, ironically, not many people trust them. The Zoons, from evidence presented in The Compleat Discworld Atlas, are ideally located to take advantage of four major rivers for the barge trade: as well as the Ankh, their home area straddles the headwaters of the rivers Quire, Koom and Sour, all of which are major waterways leading down to the Circle Sea and serving large cities and population areas. The Compleat Discworld Atlas notes they are also well represented on the run down the Quire into Quirm.