I JUST KNOW IT

from chapter 6-7 with the staircase scene and theres glowing eyes at the bottom IK im cooking

or maybe its the original form of mugo

you might be right for the simple fact he was human at first and then became a cursed god. You might be cooking too.

heh we can be cooking tg

I think the fourth mugo is an evil spirit they all fused with. All 3 mugos and the well deity call those squishy ghosts disgusting and we know they can turn into centipedes which are known in Korean folclore to be spirits that couldn't ascend and became evil. I think the mean mugo was a human who realesed the evil spirit and fused with him, the glasses mugo might have fused with them cuz he wanted power and the beast mugo might have been a child and got tricked into fusing with them or did so by accident. I also think the glasses mugo was in charge most of the time before our mc becoming the sacrifice which pissed off the fourth one especially since they are playing god while not being fully considered one and evil spirits are known to despise gods.

I think it's pretty much confirmed. The end of season 1 showed a Mugo in a hanbok. From what Sir Mugo suddenly recalled, to me it looks like he masked his true form, presenting himself to Jaehoon as Sir Mugo when he was a child (or maybe he possessed/took control of Sir Mugo?), asking him to bring sweets when he comes to them. Hanbok Mugo is the one who set about protecting Jaheon from being eaten. Remember in the final scene of S1, Sir Mugo asks Hanbok Mugo who he was while the other just laughed. So it seems the other 3 Mugos are unaware of the existence of the fourth.