Long story short, there is a legend about Gumihos where they follow some rules and one of them states they shouldn't be "discovered" as Gumihos for X days (from 100d to 3 years) by the one he's married with
When Yeon reincarnated, he still was a Gumiho but in order to become human, he had to hide it to JiAh for a certain amount of time.That's why at the end he can "predict" they'll have a daughter and he must slain Samjae else JiAh would've probably discovered the fact that it's not human yet, and if she did, he couldn't turn into human anymore
When Yeon reincarnated, he still was a Gumiho but in order to become human, he had to hide it to JiAh for a certain amount of time.That's why at the end he can "predict" they'll have a daughter and he must slain Samjae else JiAh would've probably discovered the fact that it's not human yet, and if she did, he couldn't turn into human anymore