Buttercup: Wesley, what about the R.O.U.S's?

Wesley: Rodent's of unusual size? I don't think they exist.

~ Wesley and Buttercup before being attacked by an R.O.U.S

The Rodent of Unusual Size, or R.O.U.S for short, is the quaternary antagonist of The Princess Bride.  As his name suggests, he is a giant rat who lives in the Fire Swamp.  As Buttercup and Westley recover from the quicksand, the man in black wrestles with the rodent, until he stabs him dead.