Rodents Of Unusual Size (called R.O.U.S.es by Buttercup) are giant rat-like creatures that Westley and Buttercup encounter in the Fire Swamp. They are dangerous, but only attack if they smell blood or outnumber their opponent.
Westley and Buttercup's encounter with R.O.U.S.es (in the film) was striking. The rodent's presence is made known through sinister foreshadowing before it leaps into the scene and tackles Westley. Westley immediately engages combat with the rodent and ends up having his right arm bitten, and Buttercup is horrified—standing on the side. Westley eventually defeats and kills the rodent after rolling it onto one of the Flame spurts.