The Princess Bride is a cult classic that mocks traditional fantasy archetypes and story while taking place in those worlds, making it imaginative fodder for Dungeons & Dragons campaigns. With so many resources within D&D, it's easy to adapt almost anything for a game -- including monsters. One kind of monster from Princess Bride that would work well in D&D is the Rodents of Unusual Size.
Before bringing the R.O.U.S. into the game, it's important to know where they come from. The R.O.U.S. are one of the three dangers that the hero and princess, Westly and Buttercup, encounter in the Fire Swamp. After making their way through the other two dangers, Westly comments that he thought the R.O.U.S. aren't real and is attacked by one. Supposedly, they're always hungry. In many ways, they're just D&D's Giant Rats, but you can easily take this a step further to create something unique.
In the movie, the R.O.U.S. are clearly puppets, so consider making them reanimated objects brought to life by a powerful spellcaster. Perhaps they have an extensive taxidermy collection or enjoy collecting dead things for a different kind of necromancy using Animate Object and Animate Dead spells. Instead of being a beast, this would make the R.O.U.S. objects or magical beasts. Adventurers would attack them only to knock some stuffing out, suddenly making them a formidable enemy. To make them harder for more skilled adventurers, give the R.O.U.S. a protection spell to block certain attacks.
If you decide to go the necromancy route, you could make them more horrific versions of taxidermy, stuffed with a variety of enchanted or tainted organs. Use a completely different kind of necromancer or intelligent creature experimenting with something sinister. Rats don't often travel alone, so there would be more than one, making it an interesting challenge. If the spellcaster has only one or two, then maybe they're truly larger than life like R.O.U.S.
Perhaps the reason they're such an unusual size is because the whole area they live in, be it forest or swamp, is itself an unusual size. Adventurers could have entered a strange magical forest where everything is larger than normal, either naturally or through magic, and the R.O.U.S. are part of the endemic life. The whole forest could have creatures of unusual size. The stats on Giant Rats could be altered to present even greater danger. Suddenly, that bite move could be fatal with the right roll.
In the movie, R.O.U.S. are attracted to blood, so consider adapting them as a horde of vampiric Giant Rats. Take the monster that's already available in the Players Handbook and bestiary and alter it to have vampiric traits or abilities. A bite could provide a minor health regeneration, and a large group vampiric R.O.U.S. would pose a real challenge to your party. Perhaps they're found in a vampire's lair, like Strahd's castle, or a minor one of your own creation.
If you want to go all out on a Rodent of Unusual Size, try turning it into a crazy lesser Dragon/Giant Rat hybrid. Typically, dragons don't stray in their look or types, but D&D is all about creativity and using the tools provided to create unique experiences. The R.O.U.S. could be a unique dragon that looks like a large rat with scales instead of fur and has poisonous spit or abilities that can spread disease to the party. This would make for a terrifying and awesome foe to battle. Maybe it was created by experimentation or magic, which would explain the existence of such an unlikely dragon. There are all kinds of ways to make a seemingly plain creature into something more exotic, and that's just the kind of thing D&D encourages.