Well, I’ve found something.
Courtesy of my cousin, one of my christmas gifts was ‘The Star Wars Archives. 1977-1983’ by Paul Duncan. I was just reading through it and came across this photo; as it’s from LFL archives, I doubt I’ll find it online, so I took a photo of it.
The caption for the image reads: “Han, Luke, and Chewbacca surrounded by Gamorrean guards. In the background right a man-in-lights walks by, as reference for the Lava Man creature, which was not included in the movie.”
So, it’s a ‘lava man’. They MUST have had concept art for that… what the hell does a creature made out of lava look like??