No, Skarn wasn’t in the lore prior to Diablo Immortal. There wasn’t even any references made in pertaining to him before D:Immortal. In fact, the closest reference that could be made about Skarn is that he possibly could be the father of the Colossal Golgor demons, and upon the Prime Evils defeat, he smashed half of Hell in fury. However, according to Diablo Immortal lore, Skarn is a demon of patience and subtlety, who also doesn’t act on impulse and is willing to wait for a substantial amount of time in order to slowly work situations to his favor. So I doubt he’s that demon.
So in short, I do believe that he was created just for D:I.