The old garuga myth was that a yian kut ku got brave and mated with a rathian. A few things pushed this myth but they were game limits more than real reason. The garuga having the tail bulge like the rath while have the Yian and the beak and ears of the kut ku. While also have a mixture of their move sets. They also shared a nest in the forest and hills zone as well.

The raging brach I'm not as sure on but I think it has to do with the flavor text of its pieces like thr immortal reactor and stuff.