Rodents of Unusual Size, from the 1987 movie, The Princess Bride. They are known to attack in the Fire Swamp.
-After Westley rescues her from the lightning quicksand-
Buttercup: We'll never succeed. We may as well die here.
Westley: No, no. We have already succeeded. I mean, what are the three terrors of the Fire Swamp? One, the flame spurt - no problem. There's a popping sound preceding each; we can avoid that. Two, the lightning sand, which you were clever enough to discover what that looks like, so in the future we can avoid that too.
Buttercup: Westley, what about the R.O.U.S.'s?
Westley: Rodents Of Unusual Size? I don't think they exist.
-Immediately, an R.O.U.S. attacks him-