I noticed that too but i wonder if it's just a mistranslation, on purpose or Jaheon doesn't care anymore, he was pretty heartbroken, and call them all the same.
4th mugo looks more like the original, because of the clothes, or is the animal, the 3? headed snake, god they represent.
It could very well be a mistranslation because one of the characters from Shutline (Shin’s adoptive brother) had a different name every time and it bugged me so much. All his names were: Gren Heim, Grens Heim, Grens Heims, Gren Heims. Still haven’t found out which one was the correct one?! So annoying
In the beginning I thought that the original Mugo is the 4th Mugo (the aristocratic noble one), he had a great love (the man Jaheon is a reincarnation of) but he was taken from him by the head family because they saw him as a distraction of noble Mugo’s work, so they killed him and entrapped noble Mugo in that big urn and that trauma literally broke his heart and soul into different pieces/personalities. So all the different Mugo’s represent the original noble Mugo’s different sides. But then a few chapters ago Lord Mugo said that they were created by the head family? So that busted my theory :/
then again the heir lady did say something about if mugo "remembered the whole story" so maybe he doesn't remember anything from before he was trapped in the pot. so your theory could still hold up
Jaheon used to call Horny Mugo as Lord Mugo and the cake loving Mugo as Sir Mugo, but these latest chapters he’s been calling the Lord Mugo as Sir Mugo, hmm … what’s up with that? Last time I wanted a threesome with Jaheon, Lord and Monster. Now that the Sir got enticed, I want an orgy with Jaheon, Lord, Monster and Sir! All that’s missing is that the 4th Mugo (the shadow figure from the stairs) joins in too!