Postbags are generally considered canon, unless directly conflicting with lore in-game, in which case the latter has precedence.
Thaththock the Thuthpithiouth is a lesser demon who lives under Karamja Volcano. Before the graphical update, he was identified as being the one with stubby horns and an elongated, green tongue. Due to the length of his tongue, he speaks with a lisp, evident by the transcription of his letter. He states that he does not need massive horns because his tongue is terrifying, green, and protruding.