Once you reach Chapter 6 in the main storyline, make your way to the path that lies west of O'Creagh's Run in Grizzlies East and you'll run into Hamish Sinclair who needs your help finding his horse Buell who has taken off with his wooden leg. You'll find Buell nearby, along the river to the northeast taking a drink so sneak up and get Buell under control, taking the horse back to Hamish after. As thanks, he'll offer to go fishing with you from the Veteran's Homestead.
The Veteran II
After a day has passed, make your way to the Veteran's Homestead, located along the eastern side of O'Creagh's Run and you'll join him for a fishing trip. Row the boat into the designated spot and then equip your rod with the Special Lake Lure as you begin fishing. As soon as Hamish falls into the lake, quickly help him up before he drowns and then row over to the next spot. You'll need to catch the fish yourself now and it can be tough if you're not used to fishing, consider completing the A Fisher of Fish Stranger Mission to improve your skills and then come back land the fish.
The Veteran III
Wait another full day once again and return to Hamish, this time Hamish wishes to go hunting with you. You'll get the opportunity to track the Wolf or not, this is entirely optional but if you agree to track it yourself, you can control the pace of the hunt, making use of Eagle Eye to spot the tracks. Once you come across the Wolf, be prepared to deal with an entire pack of fast Wolves so break out a weapon to dispatch them and beware of the Alpha that attacks at the end.
The Veteran IV
You'll need to wait another full day to meet Hamish for the final time and your target this time around is to hunt the Legendary Boar. Once you split up, follow the trail and it'll lead you right back to Hamish, head up to the fallen Wolf marked nearby and then as soon as you hear gunshots, rush over to the designated spot and during the Dead Eye sequence, aim for the head. Hamish is however wounded beyond aid and won't make the return trip, his final request is to take Buell which will result in an Honor increase and a new horse for you to stable.