In my campaigns, if you meet an enemy and I let you know it has the "Ancient" prefix, you know it's an old school version of that monster, complete with the classic immunities and resistances. They are often remnants of previous ages, beasts warped by ancient magics or have other, similarly impressive pedigrees.
Here then is an example - a "proper" Iron Golem! Have fun!
By the way, you may be wondering why I gave it Magic Resistance when it is already immune to spells that allow for a saving throw. The main reason is to ensure it gets advantage when saving against various miscellaneous effects and powers, and to ensure it gets a boon in those strange, usually high-level situations where weird effects change the normal rules. You can utterly ignore this if you wish!