User blog comment:WolfQueen/Discussion of the Week: Related Species/@comment-25119729-20150208184136/@comment-25119729-20150209152201

Because Black Rathian would imply it's simply a sub or rare species than an actuall new monster that has nothing in common with a Rathian other than the overall look. Also, UNKNOWNs have been proven to be off both genders, unlike a Rathian, which is always female.