OMNO

The third legendary animal is not misable. After you have completed the area of sag'Ya valley and found enough power you will be told to solve a puzzle. Solve the puzzle and you will get an orb. Place The orb stright ahead of were the puzzle is and that will create a part of a bridge. Do that two more times and you will create a the whole bridge. Cross the bridge and you will be put on this trail, just follow the trail till there is a dead end. There will be a cutscene where your character is walking then a giant jellfish looking creature comes out of no where. Jump onto the animal and and it will take you to next area of the game. When you get to the next area of the game jump off the Legendary animal and your achievment should pop. The achievment was delayed for me so I had to wait 30 seconds. If this did not help you please tell me how I can make this a better for next time I make a guide.

Note: You will not fall off the legendary animal it's scripted.