Consider the following modifications to the iron golem:
The Iron Worm: This iron golem is not shaped like a humanoid, but rather resembles a
purple worm. It has all the normal stats for an iron golem, except replace the
Breath Weapon with the
Curse of the Iron Worm:
Curse of the Iron Worm (Su): Once every 1d4+1 rounds as a free action, the Iron Worm may breathe out a score of tiny, construct worms in a 10-foot cloud cube. These worms bore into the flesh of any living creatures caught in the cloud. Those who fail a Fortitude save (DC 19) immediately take 1d4 Con damage. Regardless of whether this save is made, the worms continue to bore into the victim. One minute later, the tiny worms explode in the victim, causing 3d4 Con damage if the victim fails a Fortitude save (DC 19). The save DC is Constitution-based.
It is also has the following:
Rustproof (Ex): The Iron Worm is made of unique iron alloy and is immune to rust attacks, such as that of a rust monster or a rusting grasp spell.
I would put this monster at CR 14-15.