I modified a standard iron golem using Mike Mearls's Monster's Handbook from AEG. Monster's Handbook was published during the 3E era, but I still find it pretty useful. And it's got the Godling prestige class, which is fun.
The design was intended to fit the look of this Heroscape robot. I've got a pair of these guys and I was itching to use them.
Hit Dice: 24d10+30 (162 hp)
Speed: 20 ft. (4 squares)
Armor Class: 30 (–1 size, –1 Dex, +22 natural) touch 8, flat-footed 30
Base Attack/Grapple: +17/+33
Attack: Slam +28 melee (2d10+11)
Full Attack: 2 slams +28 melee (2d10+11)
Space/Reach: 10 ft./10 ft.
Special Attacks: Gas Nozzle, Grenade Launcher, Electro-Rod
Special Qualities: Construct traits, damage reduction 15/adamantine, darkvision 60 ft., immunity to magic, low-light vision
Saves: Fort +8, Ref +7, Will +8
Abilities: Str 33, Dex 9, Con —, Int —, Wis 11, Cha 1
Organization: Solitary or gang (2–4)
Challenge Rating: 16
Alignment: Always neutral
Advancement: 25–54 HD (Huge)
Level Adjustment: —
A Robo-Guardian is 10 feet tall and weighs about 5,000 pounds. It cannot speak or make any vocal noise save for the occasional meaningless beep, nor does it have any distinguishable odor. It moves with a ponderous but smooth gait. Each step causes the floor to tremble unless it is on a thick, solid foundation.
Gas Nozzle (Su): 10-foot cube, cloud of poisonous gas lasting 1 round, free action once every 1d4+1 rounds; initial damage 1d4 Con, secondary damage 3d4 Con, Fortitude DC 19 negates. The save DC is Constitution-based.
Grenade Launcher (Su): As per flamestrike, but half the damage is untype instead of holy. 8d8 damage, Reflex DC 20 for half damage. Useable 3 times/day. The save DC is Constitution-based.
Electro-Rod (Su): As per lightning bolt. 10d6 damage, Reflex DC25 for half damage. Useable at will. The save DC is Constitution-based.
Immunity to Magic (Ex): A Robo-Guardian is immune to any spell or spell-like ability that allows spell resistance. In addition, certain spells and effects function differently against the creature, as noted below.
A magical attack that deals electricity damage slows a Guardian (as the slow spell) for 3 rounds, with no saving throw.
A magical attack that deals fire damage breaks any slow effect on the Robo-Guardian and heals 1 point of damage for each 3 points of damage the attack would otherwise deal. If the amount of healing would cause the construct to exceed its full normal hit points, it gains any excess as temporary hit points. For example, a Guardian hit by a fireball gains back 6 hit points if the damage total is 18 points. An iron golem gets no saving throw against fire effects.
A Robo-Guardian is affected normally by rust attacks, such as that of a rust monster or a rusting grasp spell.
A Robo-Guardian’s body is sculpted from 3,000 pounds of pure iron, smelted with rare tinctures and admixtures costing at least 25,000 gp. Assembling the body requires a DC 25 Craft (armorsmithing) check or a DC 25 Craft (weaponsmithing) check.
CL 18th; Craft Construct, cloudkill, geas/quest, limited wish, polymorph any object, flamestrike, lightning bolt, caster must be at least 18th level; Price 200,000 gp; Cost 100,000 gp + 7,000 XP.