The Slither-Slider is a fictional character in one of Blueberry's mystery books.
While looking for Pupcake and Custard, the search party of Orange, Raspberry, Berrykin Bloom and Mr. Longface reach a spot where there is a bent plant. Berrykin Bloom concludes the pets walked that way. It reminds Orange and Raspberry of a mystery book Blueberry gave them - "Patty Persimmon, genius detective, and the case of the mean, nasty, berry naughty Glimmerberry thief", about a search party that was in a dark wood like the one they are in: the searchers were unaware that the thief was tracking them; however, it was not the thief, but the forest monster known as the "Slither-Slider". The story frightens Berrykin Bloom and Mr. Longface, every strange noise makes them think the Slither-Slider is around.
Eventually, Berrykin Bloom and Mr. Longface's irrational fear of the fictional monster leads to finding the missing pets: two Berrykins get stuck inside a hollow log, and the Ladybug twins unintentionally make it roll downhill. Berrykin Bloom and Mr. Longface see the log rolling in their direction with muffled voices coming out of it (the Berrykins calling for help). Thinking it is the Slither-Slider, they try to hide inside a cave - where the pets are. Thus Pupcake and Custard are found, to the delight of everyone.
- It is unknown what the Slither-Slider looks like, and what type of creature it is. The only known facts are that it is a forest monster, and always strikes in the rain.
- Unlike with the Galumphalot and the Slinking Slinker, no one assures those who are afraid of the Slither-Slider that it is only a fictional creature.