When you get access to the north of Vanaheim, to an area called the Crater, there will be a side quest called “A Stag For All Seasons,” requiring you find all 4 Stags of the World Tree.

Firstly, to get access to the Crater you need to have finished the main quest “Creatures of Prophecy” and secondly, you’ll need to have finished the side quest/favour, “Scent of Survival.” 

Secondly, in order to initiate this side quest you need to find one of four stags themselves. Once you’ve found one, you’ll then have to go back to Ratatoskr and then back to help return the stag. Do that for all 4 and this trophy is yours.
For the locations of all the stags, check out our God of War Ragnarok All Seasonal Stag Locations Guide here.