This mission in Red Dead Redemption can only be completed after you help Bonnie catch wild horses on the ranch in the mission Wild Horses, Tamed Passions. Upon arriving at the ranch, a stranger will approach you asking for help — an unknown person has stolen his horse. After talking to him, a blue marker will appear on the map. Reach it and eliminate the thief. Then throw a lasso on the horse and return it to the owner. As a reward, you will receive a few dollars and +25 points to the Honor parameter. Reach Bonnie's location to talk to her and watch the cutscene.
We learn that a deadly storm is approaching the ranch, which could lead to the death of many animals. After assigning Amos to gather a team of people, mount a horse and go to save the herd of cows. Upon reaching the herd with Bonnie, you will need to drive it to the Old Oaks. Ride behind the herd, and if any cow lags behind, prod it by approaching.
Upon arrival at the destination, you will see lightning strike an oak, causing panic among the cows. Stop them by overtaking the herd and watching for stragglers. After this, drive the herd back to the ranch with Bonnie. Completing this task will count as completing the mission, and you will receive a reward of +150 points to the Honor parameter.