Monsters specifically good against metal?

[Snowbluff]

How many of them are there? I can name rust monster off the top of my head.

[Doctor Awkward]

Dispater

...I think that counts.

[M Placeholder]

The Rust Dragon, from the Eternal Battlefield of Acheron

[Telok]

How many of them are there? I can name rust monster off the top of my head.

Delver (http://www.d20srd.org/srd/monsters/delver.htm): Corrosive Slime (Ex) on a hit.
Destrachan (http://www.d20srd.org/srd/monsters/destrachan.htm): Destructive Harmonics (Su)
Magmin (http://www.d20srd.org/srd/monsters/magmin.htm): Melt Weapons (Ex)
Black Pudding and Grey Ooze (http://www.d20srd.org/srd/monsters/ooze.htm): Acid (Ex)

[Almarck]

Lava children ignore metal and earth as if it's not even there. Metal weapons cause no damage and armor is bypassed completely.

[M Placeholder]

The So-ut (Rampager). In 2e, it was a monster unique to Dark Sun, but they put it in the MM2 for 3.5, when personally, I think it should be unique to Athas.

Its description states that it is driven barmy by the mere presence of metal, and that it has acid claws that dissolve metal. Which means that on any planet thats not Athas, its going to be insane practically all the time.

[Snowbluff]

Lava children ignore metal and earth as if it's not even there. Metal weapons cause no damage and armor is bypassed completely.

This is exactly what I was looking for. >:D

All of these are good suggestions, so I might mix them in.

[Almarck]

Well glad to be of help.

apparently the lava children come from 2e which explains their completely nonstandard ability quite well. They should be on pfsrd and if not you can probably look up Misfit Monsters Redeemed for more data.

Note I am unsure if they interact with metal at all...

[Drelua]

Lightning Elementals from Bestiary 2 get a +3 to hit and +10 on bull rush, disarm, overrun, and trip against anyone wearing metal armor, made of metal, or even just wielding a metal weapon. And they get the added benefit of looking really cool.