Story Related - Unmissable
This is unlocked close to the end of the main story.
*** Spoiler - click to reveal ***Quest: San Bakar’s Trial
After a few stops you will be tasked to find the "Lord of the Shore" which is a Graphorn. This is a boss fight, nothing too hard but you can only use Red damage spells. Watch for his attack pattern as he attacks quickly with dodge required moves. Once you beat both phases/health sections, you will be able to tame him. He also becomes your final mount you can ride. He's land only.
The achievement will unlock once finishing the map room dialogues. You should also unlock viewing all pensieve memories right before this.
Edit: After completing this mission you also unlock the only spawn location for Graphorns in the game. You can visit the den after completing the trial and fight a graphorn in order to tame it, this is needed for the breed all beasts achievement. Only beast in the game that you have to fight in order to tame.