Actually, Alpha vampire said : We all have mothers. Then we were smart enough at this board to deduce there was a mother to all monsters, which was a good assumption, but still very hypothetical. So we called her Mother of all Monsters at this forum.
Then she was given title 'Mother of All' in the episode with the dragons. But I share Libs' opinion that it's really 'Mother of all Monsters' and not Mother of All Living Beings or whatever.
I had forgotten that the title Mother of all Monsters had only been used here. That's how ingrained it's become. Still weren't we the first to use Yellow Eyed Demon? That ended up in the show.