Ancient Builder: "New" Monster

Old Ones with Shoggoth, art by Howard V. Brown for At the Mountains of Madness
Astounding Stories, Feb 1936 issue. Source.

During the decade or so of time that Holmes wrote for and about D&D and RPGs (about 1976-1986), he didn't provide us with many "new monsters" that are not found on the standard Monster List of Basic. Most notably, he introduced the Were-Shark into D&D, and there are a few non-standard monsters in the Sample Dungeon, mostly giant animals, such as the giant crab, large octopus and giant snake, each given partial stats and description, and which serve to demonstrate a new DM that they are not limited to the standard monsters. 

Turning to magazines, we can look to Alarums & Excursions #18, published in January of 1977, for Holmes' adaptation of several Pellucidar creatures from his authorized sequel, the Mahars of Pellucidar, including the intelligent pterodactylan Mahars, their ape-like Sagoth servants, and the giant, flying Thipdars. And his most extensive write-up was for the article "The Lovecraftian Mythos in D&D" in Dragon #12, published in February 1978. While co-author Rob Kuntz wrote the entries for the deities Cthuga, Ithaqua and Yig, Holmes was responsible for the rest of the deities, as well as all of the "monsters": Byakhee, The Deep Ones, The Great Race, The Old Ones, The Mi-Go (aka the Fungi from Yuggoth), and The Shaggoths [sic, for Shoggoths]. These monster entries were later revised and partially re-written by Jim Ward for inclusion in Deities & Demigods, where The Old Ones were retitled The Primordial Ones to avoid confusion with Lovecraft's deities, such as Cthulhu, that are also referred to as Old Ones. These creaturesare alternately referred to as The Elder Things by Lovecraft and the Call of Cthulhu RPG.

The intro to the Sample Dungeon, the ruins of the tower of the wizard Zenopus sit upon "the ruins of a much older city of doubtful history" which is several paragraphs later referred to as a "pre-human city". And then in his D&D novel The Maze of Peril, first published in 1986, Holmes writes of the origin of the vast underworld: "Somewhere beneath the surface of this ancient land the tunnels and corridors of some prehistoric race coiled and raveled, delved, and probed unimaginable depths into the core of the world" (pg 3). Holmes does not further disclose the nature of this race, but Lovecraft's Old Ones are a natural fit, being both pre-human and extensive builders. Here's my analog of the Old Ones where they fulfill this role of original architects of the underworld, written up in Holmes Basic format. It also mentions Dagonites and Whisperers, which are analogs for The Deep Ones and the Mi-Go, respectively. 

Ancient Builder

Move: 120 feet/turn

Hit Dice: 8

Armor Class: 5

Treasure Type: 50% chance of 1 weird device with the power of a ring or wand

Alignment: neutral

Attacks: 1d4 leg tentacles

Damage: 1d8 points per tentacle

Sages speculate that the vast underworld was the effort of many races, toiling over the eons, but that it began with vast underground cities built by a pre-human race that came to the lands from regions unknown. This race of Ancient Builders flourished for a time but was eventually weakened by wars with the malign Whisperers, and ultimately overthown by their own creations, the now mindless Black Puddings. Over the ages their cities were much changed and expanded by later races such as the Dagonites and Troglodytes, but there are still occasional reports of adventurers finding hibernating Builders far below the surface.

Extreme caution is urged as these sleepers may lash out at intruders if awakened or capture them for further study without care for their well-being.  

The Builders are said to be slightly larger than man-sized, but wholly inhuman in form, with a columnar body with a ring of tentacles at the top (sense organs) and bottom (legs), and more at the sides for grasping and holding items. From the relics of their culture it is clear they are highly intelligent albeit in fashion alien to humans, and they often wield weird devices that duplicate the powers of rings or wands (1d20 charges).

Update: 

The Ancient Builder now appears under the name "Old One" in the Blueholme Journeymanne rules, available at DriveThruRPG.