Board Thread:General Discussion/@comment-6192846-20150822000535/@comment-6986530-20150822005424

It isn't stated Gudonechia is in the First Period, just that it's where Behemo was born. There's also an indication that their human forms are from the Second Period, while they could have been something else in the First Period and reincarnated.

I'm also going to state that sometimes we word things on the wiki a certain way even if the text itself does not, because plot twists were not taken into account. There's a lot of leeway for this song.