The Giant Gila Monster

Poster for the 1959 film "The Giant Gila Monster" directed by Ray Kellogg

Above is a poster for a 1950's monster movie, The Giant Gila Monster, which may have served as inspiration for Dr. Holmes for a D&D monster. I haven't seen the movie yet, but there's a MST3K version on Netflix which I've added to my queue.

On page 38 of Holmes' 1981 book Fantasy Role-Playing Games there's an untitled dungeon map which we have been calling the "Halls of the Lizard King" based on a key feature. It's just part of a dungeon level, and there's no corresponding text describing the dungeon, but the map itself provides descriptive names for many of the rooms and features. Notably there's a colossal cave housing a "Giant Gila Monster". According to the Wikipedia entry, the monster in the movie is about 70' long, and which is similar in size to the creature on the Holmes map. If the squares are 10' each as on another map in the book, the monster is at least 100' long:

Detail of Holmes' map showing the 100'+ monster

This map likely came from Holmes' own campaign. Since we unfortunately don't have any other information on his version of the monster, I've written up my own take on it for Holmes Basic. Hit Dice are extremely high since Purple Worms are 50' long and have 15 HD. The poison breath is inspired by an Apache legend, and the healing properties of their skin from Sonoran Indian legends.
 
Giant Gila Monster

Move: 30 feet/turn
Hit Dice: 20
Armor Class: 3
Treasure Type: see below
Alignment: neutral
Attacks: 1 bite
Damage: 3-18 points

Enormous burrow-dwelling predatory lizards, up to 100’ long when fully grown. Their breath is so foul that all creatures in a thirty-foot radius must save versus poison each round or take 1d6 points of damage. They are slow-moving but bite quickly, and will grip their victim tightly, inflicting 3d6 points of damage per round. Even worse, their salivary glands secrete a toxin that requires a save versus poison each round to avoid taking an additional 2d6 points of damage from pain. They do not hoard treasure but their hide is valuable (1d6 x 10,000 gp) for two reasons. The hide has regenerative properties; ingesting a portion will restore 1 extra hit point at the end of the day, and magic-users can use it to brew healing potions. Furthermore, the tough, beaded skin can also be used to make colorful armor equivalent to plate mail but light as leather armor.

Edit: Revised 1/17 to add properties of hide.