Way back when Eberron first debuted, one of the many innovations the setting brought to Dungeons & Dragons was living constructs known as warforged. They were golems you could play, or to put it another way they were basically fantasy robots. Sculptor Ben Siens does an admirable job of mimicking the warforged appearance with the serial numbers filed off, which is how we end up with a "battleguard."
The golem has been with Dungeons & Dragons since the original Greyhawk supplement:
Golems are magical constructs made of inert matter animated by high-level spellcasters through the use of powerful spells during magical rituals. The main aspect differentiating one type of golem from another is the material from which it is built. There are four standard types of golems, from weakest to strongest: flesh golems, clay golems, stone golems and iron golems.
This golem probably qualifies as an iron golem. One point of note is that the golem's size is large, which means if you plan to use it as a warforged it will need to be a warforged charger.
The roots of the iron golem go beyond warforged however, back to Talus, the iron knight of Edmund Spenser's "The Faerie Queene":
Talus’ other superpowers include speed (“him pursew’d so light, / As that it seem’d aboue the ground he went”), invulnerability (though a bad guy “streight at him with all his force did go,” Talus was “mou’d no more therewith, then when a rocke / Is lightly stricken with some stones throw”), and strength (“But to him leaping, lent him such a knocke, / That on the ground he layd him like a senseless blocke”)
The invulnerability might explain the iron golem's damage reduction. But for the poisonous gas endemic to iron golems we have to go back further to Talos, the bronze man of Greek myth. Talos was a bronze giant forged by Hephaestus who had a single nail that plugged up a vein that ran from his ankle to his neck. The original Talos was large but not the size of an iron golem -- it's pretty clear Ray Harryhausen's version from "Jason and the Argonauts," and for evidence look no further than the original "Monster Manual" illustration where the iron golem is clutching a Greek warrior in one hand. It even has the same helmet!
You can purchase this miniature at Amazon.
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