Enter camera mode (press  ) then select Black & White Filter (if you didn’t already, unlock it using 150 confetti), then exit camera mode. Find a gopher and an elk. Once you find them, pick up the gopher and throw it at the elk, if you did it correctly the gopher should ride the elk. Now enter camera mode and while in black & white filter point the cursor at the gopher and elk, take a photo (press  ) and the trophy should unlock.

If you’re looking for a place for this, then at the end of Chapter 3: Maypole Fields, Chapter 4: St. Swithin’s Moor or Chapter 5: Grocer’s Bar you’ll visit a large open area. In the left part of the area, where you find the entrance to Chapter 6: Gibbet Hill, is a snow area with a few elks. If you search the entrance to Chapter 4, you should find a few Gophers. There are 3 that you have to collect in order to open the blue present near a cage, one of these Gophers is near the place where the snow stops.

If you're having trouble here's a video:

Good Old Days Video Guide

Credit to Joshi_Marston for the video.