The iron golem is one of the tougher golems. Their breath is poisonous and can drain your strength. On creation, they have a chance of a battle axe or a two-handed sword or a bow and 3-14 arrows or a longsword or a lucern hammer or (64% chance) nothing. They are not eligible for a random offensive item.
Lightning slows them, and fire heals them - possibly by welding in their dents. They are hurt by rust monsters. Rust traps will instantly destroy them. If you are polymorphed into an iron golem when you step into a rust trap then you will take damage equal to your monster form's maximum hp, killing your monster form and reverting you to your natural form. The half physical damage extrinsic applies.
When killed, an iron golem's corpse will be some number of the infamous iron chains, widely considered one of the most useless items in NetHack - although some players do find the occasional odd job for one. As iron chains are quite rare, it is worth consulting that entry when you dispatch an iron golem.