hunting large monsters would produce large bones. no specific wyvern or pelagus gives that.
Fight large monsters!
You get a large monster bone from the large monsters like the kut-ku.
Large Monster Bones vary from different monsters. You can get them from monsters like the bulldrome, Rathalos, Rathian, and other large monsters. Monsters like apotonoth, popo, and other monsters like them do NOT give large monster bones. For more information, visit monsterhunter.wikia.com
you can get it in the jungle map number.5 there will be a large monster only need to hit about 4 to 5 hit if you using bone katana or iron katana and when you killed it you will need to press(circle) and some time you are lucky you will get monster bones or raw meat!
mining point 3 4 5 of your farm. bomb mining cat with large barrel bomb or large barrel bomb plus.
5th mining point in your farm, give you Bomb mining cat a Large Bomb + to get that. mining points in G Rank missions.
in the desert, look for a large monster with a spiked tail. kill these!!! ---- they are not hard and you can carve them twice.
try killing large wyverns e.g akantor/lao shan/shen and sell the parts fatalis though seems to make the most for me
Whenever you beat a Cephadrome there is a large chance you will get one at the end of the mission.
G rank mining spots, highest ranking mining spot in the farm bomb mining cat with large barrel bomb plus provided you have the highest mining spot renovated also.
You press circle on gathering areas, fight large monsters or gather at any of the old areas (old Jungle, Desert, old volcano and even the Great Forest). Or fight monsters that are high rank, or G rank.
Monster Keenbone is randomly dropped from the following large monsters (High-Rank, 6 Stars+ only): Anjanath (High-Rank, Ancient Forest) Barroth (High-Rank, Wildspire Waste). ☺