I have some tips for if you plan on fighting a Rath-Dagon for your first time.
Both have fire attacks, but the Rathian's is less intense.
Both have a poison attack:
A Rathian can flip when flying, so contact with her tail will cause poisoning.
A Rathalos will sometimes try to scratch you when flying, so contact with his claws will poison and "arouse" you, causing confusion as well.
A Rathalos will sometimes waste a few minutes by flying around an area during battle. Use this as a time to use a wetstone, flute, BBQ split, or some other item.
Damaging wings badly can get you parts of them as items.
Rath-Dragons will change their minds. If they start to fly to go somewhere else when they don't see you, they will go back and land if they DO see you.
Rathians are easier to fight, so be MORE careful when near a Rathalos.