Gaspode the Wonder Dog is a small, greyish-brown terrier who suffers from most doggy diseases and a fair number of diseases that aren't doggy, including Licky End, which normally you only get if you're a pregnant sheep.
Due to several unfortunate magical incidents, Gaspode is a good bit smarter than your average dog, or indeed human, and can talk. Of course, most humans never notice this, because as we all know, dogs can't talk. In fact, Gaspode goes unnoticed a lot of the time, which is why he has known of PPC activity for quite a while; Sues in Ankh-Morpork usually forget he's around, and that he knows almost everything that goes on in his city. Because Sues don't mention him, PPC agents usually don't see him on missions. However, that doesn't mean he doesn't see them, as he explains to Trojie and Pads in 'Out in the Rain'.