That last turn, a fiend was FIELDed, and using its MOVE, it stepped onto the same space as another fiend, automatically merging.
We don't know how many fiends are in their reserve, and I REALLY don't want to find out.
At least if we take out the big one now, we'll only have to deal with either another 8/6 next turn or two 4/3 fiends. If we drop the small one, they'll field a fiend, and it'll likely merge with the big one and become a 12/9 beast. 12/9 is far too stompy for my taste.