It was mainly the sword and environment that made me think it was for Surtur. I wouldnt be too happy with that, no, but that could be why it wasn't used and was only 'concept art'
For MCU purposes, they might not make him quite as big, and they may not make him look exactly the same. It sounds like Kurse is going to be quite different from the comics, and may not be quite as big either.
I was thinking the other was a fire demon, if it was meant for Thor/Thor 2.