Talking to Follows-Chalk about interesting wildlife in Zion will initiate this quest. After receiving it, head to Angel Cave and go up the slope to the left of the camp as you look at it from the island. After maneuvering across the river over the three rock bridges, you will find the Lost Mountain bighorner calf at the end of a narrow passage opposite the end of the last bridge.
In order to complete the optional part of the quest, just don't get too close to any of the other bighorners and they won't attack you. If you do get close, make sure you back away as soon as you can, as they won't attack if you move quickly. If the Courier, a companion, or any other non-player character kills a bighorner, the optional part of the quest fails.
You will have to feed the lost calf a banana yucca fruit to get it to follow you. Make sure you have 3-4 banana yucca fruits on you (there are 3 banana yucca plants along the route) as the calf will periodically stop and you will have to feed it another before it will continue. It can take 5–10 seconds after feeding for the calf to resume following you again.
Return back across the rock bridges to the bighorner mother. Choose to either return the calf or keep it and if the former, speak to Follows-Chalk to complete the quest and receive the reward. If you killed any bighorners in the process of completing the quest, Follows-Chalk will still give the reward. He indicates that he understands, but wishes they had not been killed. If you kill no bighorners, you will receive 25% more XP for the quest.
Once you have found the lost calf, you may want to save your game. There are a number of random ways this quest (or just the optional part) can fail, so you may need to reload and try again.
The bighorner calf can fall off the rocks when it's following you, making travel difficult.
If a bighorner strays into the Dead Horses camp, the non-player characters may kill it if it becomes aggressive, causing the optional part to fail.
Having the Animal Friend perk will make this quest considerably easier.
A giant cazador may appear and attack the bighorners which could cause you to fail the optional part of the quest.
Successful completion of this quest yields a .45 pistol and a war club, both in perfect condition.
Occasionally, a Dead Horses tribe member may wander up the path leading to the bighorners herd. When this happens, he or she may attack the bighorners causing you to fail the optional portion of this quest.
Don't forget to make your follower (if any) become passive, if not, they will attack the bighorners on sight.
If you kill the bighorner mountain mother before the quest, you can still take the quest and successfully complete it after returning the calf and talking to Follows-Chalk. Also, after the quest you can kill the calf for XP, if so desired.
Followers such as Follows-Chalk or Waking Cloud may be spooked and forced to start killing bighorners, which will fail the optional part of the quest.