A Look at Monster Hunter Portable 3rd's "Intrusion" Quests
Powerful beasts attack when your main quest ends.
The proper English terminology for Monster Hunter Portable 3rd's recently announced "Rannyuu Quest" system appears to be "Intrusion Quest." That's the URL Capcom has chosen for the component's page at the MHP3rd official site.
Intrusion Quests are quests within quests. Some quest are marked as "Unsafe Hunting Environment" (the orange text in the above image). For these quests, after clearing the main quest goal, a warning message will sometimes appear on the screen. This means that a new beast has entered the field, and you'll be able to take him on using your current life, stamina and items.
You don't have to take on the new beast if you don't feel like it. After the Intrusion Quest starts, you can go into the menu and select to head back to the village. You'll get to keep all the rewards from the main quest.
Continue with the new hunt, and you'll find that some new support items have been delivered to your camp. You'll also have a new clock, giving you have ample time to take on the beast.
There's actually no penalty for failing at the Intrusion Quests. Even if you get beaten by the beast, you'll still receive the rewards from the standard quest.
It looks like the only thing that may end up getting in your way is if you find yourself afraid of giant Intrusion Quest beasts like Deviljho: