The monster parts you harvest after defeating a creature in Monster Hunter Rise will typically go towards being made into a new piece of armor or weapon. When you enter the game’s high-rank missions, there’s a hard reset you have to go through, and you begin the steady climb of battling old monsters that have a lot more health and strength behind their hits. There are also a few new monsters for you to encounter as you progress. A notable resource you want to add to your inventory will be Monster Keenbone, and you’ll be using it to enhance your weapons.
Monster Keenbone typically drops off of several monsters in the high-rank missions. Barroth, Khezu, Jyuratodus, and Rathian drop it. Of the four, Barroth is probably easier for you to battle when you’re starting to stretch your legs in the high-rank missions. If you and a few friends are ready, Khezu could be doable for you to battle, but Jyuratodus and Rathian are likely the more difficult encounters.
We have not seen any of the small monsters dropping this item, so if you’re searching for it, you’ll have to undergo one of the larger hunts. You can also go out of your way to pick an expedition mission and hope one of these monsters appears in the region you’re hunting if they appear there. Khezu only shows up in the Frost Islands and Lava Caverns. Barroth only shows up in the Sandy Plains. Jyuratodus will only appear in the Flooded Forest. Rathian appears all the place, in the Shine Ruins, Sandy Plains, Flooded Forest, and Lava Caverns.
Unfortunately, Monster Keenbone has a fairly low drop rate. It’s only available as a target reward, and it has about a 10% chance of dropping. You need to pick whatever monster you want to hunt, go after it in a high-rank mission, and hope your luck pays off at the end of the mission that the item drops.