Rust Monster

Iconic as they may be, rust monsters are quite bad. 5 aberration HD are the kind of thing that need to be compensated for by unique and powerful abilities. Sadly they lack them.

The ability scores alone are bad enough. No net gain, and -8 intelligence are already sucky, especially when the lack of strength is taken into account (not much caster potential here). The bite attack is basically only useful for mouthpicks, and the natural armor is honestly poor for a 5 HD monster.

Scent, with the added ability to smell metal objects up to 90 feet away is not too special either. Rust itself is another fancy way to destroy your own loot, though I admit that a fair number of low-level dungeon features can be bypassed with it (though something like a blink dog could do the same, plus more).

As is perhaps to be expected of an one-trick CR 3 monster with 5 HD, the rust monster deserves -0 LA.