The Lord of the Mountain is a secret mount you can ride in the game.

In the Sanctum of Hyrule Castle, around the triforce is Zelda's Lullaby as it appears in Ocarina of Time

The trousers of the hero have a little octorok from Zelda NES on them.

This page will list all the Easter Eggs, hidden references, and other secrets in the game The Legend of Zelda: Breath of the Wild.