'Enter, Pursued by a Memory' is the second mission of the entire game. It is available directly after the first mission 'Outlaws from the West'
In this mission, you follow Javier and rescue John from wolves on the mountain, after some scripted sequences you find John and put him on Javiers horse. You must kill 2 packs of wolves, one pack while putting John on horseback, and another pack while riding to camp.
(Video guide below for a gold medal)
(Not made by me, but by the wonderful GTA Series Videos)