Starting as a toy that got stated up by Gary this monster is the bane of heavily armored PCs everywhere. Relatively harmless if not for its unique ability to rust any metal that comes into contact with it. Iconic and completely known to anyone who has been playing D&D for a time so of course this is ripe for using player knowledge against the PCs.
First up all you need to do is have the threat of one to affect the party. Rumors of a strange monster insect thing that rusts all that touches it will be enough to get the fighters sweating in their metal tin cans. An illusion of one in a maze can delay a party for quite a while if fighter heavy as they try to find another way through the maze. Doing this you don't even have to have a Rust Monster there so if your a nice DM you don't have to take away your players shiny new plate armor. Of course if you have a reoccurring villain who does this enough that they catch on, then when they finally get to his main lair having a real one will throw the party for a loop as well.
Another way to use them is having them be down a non-essential corridor with treasure at the end. You may find that the players will avoid it even if they know about the treasure. Of course beware, the easy answer to this is have one of the non-metal wearing party members lay down a trail of metal away from there to guide the Rust Monster away. This should work because they are only animals. A trail of food works just as well with guard dogs that are not chained.
In actual combat the Rust Monster should care only about the metal that is being carried if attacking of its own will. Once hit by the party it will go after them but before that its only a hungry animal. This does not mean you only have to attack the fighter though. If it is coming from behind the party it would attack who ever is closest with metal and if the Magic User in the back has a dagger, the last I heard daggers where metal.
To finish this up I will remind you all that part of how big an impact something has is partly also about how unique the experience is. If every other encounter is against a Rust Monster then all that will do is have the party looking for non-metal gear. A once off encounter when they least expect it can really put the fear in them. Not to say you should only have one Rust Monster in your world but that it should not be a common encounter. Their diet means that they have to live somewhere that has a lot of exposed metal for them to eat. An old abandoned iron mine is more likely to have them then a regular cave.