アンタラスの監視者ギルモア:
Then prove it with your skills, not words! You'll get a Large Dragon Bone when you defeat the monsters in this region. Bring the bone to one of the Dragon Votrexes found in many places in the Dragon Valley, You will meet one of the servant of the land dragon.
Kill a Emerald Horn, Dust Rider, Bleeding Fly, Black Dagger Wing, Shadow Summoner, Spike Slasher, or a Muscle Bomber and bring their Large Dragon Skull.
Of course, that means you'd have to kill each of them at least once.