If you want to know how to get to the secret ending in The Last Guardian, then just watch the final credits roll to the very end. Just be patient and soon you will see the post-credit cutscene that actually reveals a few interesting moments.
To begin with, we see a grown-up man picking up a magic mirror from the ground and being able to use it. Secondly, he is all covered in those peculiar tattoos that we could see on the boy -- the game’s main protagonist. Does this mean that this man is the grown-up boy who survived the journey in the valley? Many fans of the game believe it to be so.
But that’s not it! At the very end we see Trico’s eyes in the shadow and a familiar growl. However, the last few frames show another pair of eyes glowing besides Trico. This could mean that he found a mate for itself. But most importantly -- it means that Trico is well and alive.