I tried three times to FD split him from the two stonemight elders with him. I was unsuccessful everytime. When Fd'd the elders stayed with him and never moved until he did - which made for a long wait.
Spawn location was: -1560, 801. He looks like the other goblins in the area, only slightly larger. Couldn't get a clean screenshot due to a multiple pull. He does see invis.
There is a small stone hut at that location where Stonemight Chieftain Rockbottom is located. He was one of the goblins located outsite the entrance of the hut, so presumably any of those could be the placeholder.
Loot: Smash Goblin Parts Amber A Stonemight Ear
When I pulled him the 2 goblins nearby came with him, even though they had been pacified with Nature's Placidity, so they appear to be linked.
Duoed with a 75 Pal/75 Dru. The mobs path weird in this area, probably due to the terrain being rough. They will "warp" short distances during the fight.