Story-related, cannot miss.

Late on in the game - the 38th out of 40 main quests, in fact - you’ll need to find a certain beast to gain access to San Bakar’s trial in the quest aptly named, “San Bakar’s Trial” quest. Yes, that beast of course is a Graphorn, and yes, he’s dubbed the Lord of the Shore.

Basically, head to the quest marker and fight him, and then capture him at the end. It's as simple as that.