User blog comment:Kogath/Monster Hunter XX Discussion/@comment-26532315-20161027164501/@comment-26261665-20161028145153

That still doesn't confirm that any monster beat King Diablos. We know about a hypothetical Red Diablos since long time ago because of BannedLagiacrus' blog, but still, anything says that the King Diablos lost again a red one. You can interpret it that way, but still...

The MHFU quest onlu speaks about King Diablos btw. It doesn't say anything about it being besten by another monster.