This entity appears very briefly in Witches Abroad.
It swims up to the boat containing Magrat Garlick, Nanny Ogg, Granny Weatherwax and some Dwarf Bread, and announces, apropos of nothing, that today is its birthday. For its pains, it gets clobbered on the fingers with an oar by Granny Weatherwax, who watches it recede into the distance and listens to its swearing complaints dwindle to nothing.
The witches agree this was clearly a "troublemaker", and Granny ponders on what it wanted.
Were it not for the threat of invoking the attention of one of the most potent and dread forces operating anywhere in any fantasy fiction universe (touches iron, and nervously whispers The Lawyers of the Estate of J.R.R. Tolkien. Notes sword is not glowing blue: it is safe to continue) the casual reader might wonder if this is Gollum, from The Lord of the Rings, paying a passing visit to the Discworld. However, this is NOT a visit or a cameo role from a character licenced exclusively to New Line Cinema and the Estate of J.R.R. Tolkien, and anyone mistaking this random character for Gollum(TM) is in fact seriously mistaken. Honestly. Just think of this character as the Frog-Like Humanoid Thing with a Birthday. It will be safer, cheaper and less litigious this way.
The 1995 film Dracula: Dead and Loving it has a scene where Dracula (Leslie Neilson)is trying to fly into Lucy's bedroom, only to be concussed by the shutter as she closes the window..... hmmm. Can't be plagiarism, as in this book (1991) Magrat concusses her vampire by opening the shutters as he tries to fly in...
the whole [Happy Valley] fairy-tale parody in Monty Python's Flying Circus....