Gnasher

According to www.beano.com, "Gnasher is an Abyssinian wire–haired tripe hound – this is a very rare breed of dog which comes from the mountains of Eastern Africa. Abyssianian [sic] wire–haired tripe hounds are world–famous for their thick black coats which are as strong and tangled as barbed wire, and their especially strong teeth, which can chew concrete and smash bricks (not to mention what they can do to a postman's trousers)."

It would be a very strict question master that didn't allow "tripe hound" as an answer though.

It hardly needs saying that in the real world there is no such breed as a tripe hound (of any type, let alone the Abyssinian wire–haired variety). The single word 'tripehound' does however have a life outside Dennis the Menace; according to Wiktionary it was first applied to triplanes in the First World War (due to the similarity of the sound). Wiktionary cites both the German Fokker and the British Sopwith as models to which the name was applied. If you search for 'tripehound' in Wikipedia, it redirects to the Sopwith Triplane page.

Later, the word came to be used to mean "An unpleasant or contemptible person; also ... a newspaper reporter or an informer" (Oxford Reference). It was apparently used in 1923 by the New Zealand crime writer Ngaio Marsh.