The Sulfur Grendels occupy Grendel slot S and were created using Geneforge/Eggforge/Gimp.

They come complete with a modified CFE Grendel genome that improves their resistance to heavy metals, makes them less inclined to beat up norns, and instinctually seek out critters for food.

Be sure to install the bigger sprite allocation fix by Verm, otherwise your game will crash when they reach adulthood.

https://creatures.wiki/Bigger_Sprite_Allocation