Sorry, but I'm currently fixated on Caverns of Thracia at the moment.
Regardless, this creature entry--#68, on p.66 is a very curious inclusion.
1. It differs much from the descriptions found in the Blackmoor supplement and AD&D monster manual for Giant Lizards. 2. It is not even described as being "giant-like". 3. Yet it can inflict massive amounts of damage via 3 attacks (bite: 4-24/claws x2: 1-12 ea.)
Clearly Jaquays concocted this creature for the scenario, regardless of the source inspiration. But, physically, it is one of the most powerful things in the adventure.
Both it and The Incarnation of Death (Rooms 9A-13) add a unique element to the adventure; a benefit, I believe, of JG's design approach.
But what category would you place the creature within?