When you're done questing around, launch an expedition to the Ancient Forest and start searching the area for mysterious tracks, which will increase your Scoutfly Level and allow you to hunt down whatever left these tracks just like any other monster. Find enough to track them, then follow them until you reach AREA 16, the Rathian nest atop the Ancient Forest. Here you'll be introduced to another flying beast, the Rathalos, which proves itself to be just as belligerent as the Rathian, but far, far more dangerous a foe.
After barely escaping with your skin, you'll complete your objective. Unfortunately, the First Wyverian is not impressed with your combat prowess, and declines to assist you until you prove yourself worthy... this allows you to start the assignments A Fiery Throne Atop the Forest and Horned Tyrant Below the Sands. That's right, your next two tasks involve killing the Rathalos and the Diablos. Looks like you've got your work cut out for you!
Now is a good time to attend to any optional quests you may have left to do, as you're going to need all your resources to tackle these two monsters. Be sure you have Demondrug and Armorskin items to buff yourself, as well a Max Potion to max out your life bar and provide superior, quicker healing to other alternatives. Also be sure to have a defense score of over 150 - anything less and you're asking to get killed in three or four good hits, even with a maxed out health bar.
For the Rathalos, consider cobbling together three items with the Poison Resistance skill so you won't have to worry about Antidotes during the fight. There's enough going on already with the Ratholos's fire breath and physical offense. Speaking of which, having some fire resistance certainly won't hurt, although it's a secondary concern. Just make sure you don't have a negative value and you should be... well, good enough. There's only so much you can do to defend against the Diablos, but it relies heavily on physical attacks, so defense is extra important. The Rathalos is vulnerable to Dragon element attacks, while Diablos can be more effectively fought with an Ice elemental weapon or a Paralysis weapon.
Prepare yourself accordingly.