The mission becomes available after completing Chapter 6 in the RDR2 Story Mode.
The Veteran Mission Walkthrough
The Veteran - Part I
An old outdoors man and war veteran is thrown from his horse and has it recovered for him.
You will find this stranger seated against a rock by O'Creagh's Run, to the west of Annesburg. After your conversation, run toward the highlighted area to the east. You will soon see the man's horse, Buell, by the lake.
Walk to him while holding L2/LT and press Square/X as prompted to calm and pat him. Once he has accepted you, lead him back to the veteran - who then introduces himself as Hamish Sinclair.
The Veteran - Part II
Hamish takes his new friend fishing, entering an epic battle with a goat northern pike known as The Great Tyrant.
Available between 6am and 6pm only. Visit Hamish Sinclair at his cabin.
Once inside the boat, row to the highlighted area, then equip your rod with the special lake lure and start fishing. When your companion falls in the water, press Triangle/Y to grab the oars and row to him. Row to the fishing spot and set up again.
This time, the Legendary Northern Pike will bite - and give you a serious challenge. To succeed, boost the efficiency of your reeling sessions by regularly tilting the Left Analog Stick downward to pull the rod up.
The Veteran - Part III
Hamish and his friend attempt to track a wolf, only to encounter more than they bargained for.
Available between 6am and 6pm only. If you pay Hamish Sinclair another visit al his cabin, you will soon find yourself on the trail of a wolf.
When Hamish asks you if you want to track the animal yourself, feel free to accept his offer: Eagle Eye enables you to follow the trail at your own pace. When you approach a wolf at the end of the path, be ready for an ambush in which an entire pack will attack you. They move fast and can reach you within a split second.
Should one manage lo bite you, unleash a swift melee blow followed by a volley of hip shots. Be ready for an automatic Dead Eye moment when the leader of the pack rushes toward you.
The Veteran - Part IV
A huge wild boar is sighted near Hamish's cabin, and a hunt begins.
Available between 6am and 6pm only. You can visit Hamish Sinclair one last time at his cabin. After the cutscene, go after the Legendary Boar until its trail leads you back to Hamish.
Mount up and inspect lhe marked dead wolf at the top of the nearby hill. When you hear gunshots, gallop to the highlighted area until you find Hamish again. During the automatic Dead Eye moment, tag he boar's head multiple times to take it out instantly.
Don't forget to hold Triangle/Y when close to it to take a trophy from the animal. You can also claim Hamish's horse Buell at this point if you wish, or al least take him to a stable to keep him safe.