Chocolate Gumby started as an attempt to cross El Choco and Gumby, two isolations of Dave Wombat. The grow of that cross attempt became contaminated, but Dave managed to save some cool little fin mutations that popped up in that tub for cloning. That mutation just got weirder in the following generations. Forming blobs, popcorn shapes, and partial caps. 
That original fin mutation had never appeared in grows of either of the parent genetics. So it is impossible to know if this was a mutation of El Choco, or Gumby, or Chocolate Gumby mutated by contamination. But due to all that complicated history, Dave just named it "Chocolate Gumby Brains".