This dude is one of Snatcher's minions. He makes a fuss on the cheese hunt that Arthur observes, and while he's acting as one set of horse legs, he tells the rider "'Well, I'm not going on, and I'll blame you two if we get in trouble for getting left behind'" (2.12) as a way to get his turn on top. Sounds a little passive-aggressive, don't you think?
He's one of Snatcher's main goons, since he gets tasked with things like lowering the cage of cheese into the Fondue Pit (6.5) and selling off mini-critters (11.5). When he visits Willbury to sell some critters, we get a better look at him: he's a "rather grubby man wearing a frock coat and top hat" (11.4), and he has an "oily voice" (11.5). Sounds perfectly trustworthy, right? Not.