I have a feeling of the same as well. That it's the Reptus Hulk. It can't be the nightmare zone, as once you are done with Dream Eater, you can't re-enter the zone ever. However I believe, its the Deep Marsh. Seeing the amount of people hunting it, if it was in regular fester map, I think it might have been caught by now.
Once Deep Marsh opens again, we will know