Gold Rathian are rare genetic mutations of the Rathian.
She can be found mainly in the Towers along with unexplored parts of Fonron though they will leave the Towers to hunt in other areas such as the Old Swamp and Misty Peaks.
Just like Pink Rathian, her fighting skills are more advanced most likely from her color or possibly part of her genetics. Her power is actually close to those of Elder Dragons.
Gold Rathian seems to prefer living in areas not to warm from her hide being able to absorb heat very quickly.
All three of the HC Rathian, seem to be old individuals.
Like Rathalos, there is the Old World Rathian and New World Rathian.
Its believed that the Rathian in Val Habar might be a 3rd Subspecies of them.
Rathian Shards are thick Rathian Scales that are large in size showing that they are old in age.
The thorns on her back also contain poison.
Gold Rathian scales when reflected off sunlight give her an almost sacred glow. Gold Rathian thorns are able pierce through anything.
Why is it an Elder Dragon-Level Monster?
Rare Species are about as rare as Elder Dragons. Some Rare Species even have some abilities that is on par with Elder Dragons. It has even been found that some Rare Species use the Dragon Element like Elder Dragons, though isn't really noticeable most of the time.