Horizon Forbidden West has 17 Rebel Outpost Locations. Finding and completing all least 4 Outposts is needed for the trophy “Completed 4 Rebel Outposts”. They are smaller versions of Rebel Camps. You need to defeat the Outpost leader and loot his body for Rebel Tags, then leave the area. When done correctly the Rebel Outpost icon will turn from blue to green on the map.
You can also track Rebel Outposts under Quests > Rebel Outposts. There it will show all the ones you have discovered.
None of the Rebel Outposts are missable. You can still do all of them after the story in free-roam.
For the bigger Rebel Camps, refer to: Horizon Forbidden West – All Rebel Camp Locations.
Rebel Outpost Locations
Rebel Outpost #1: The Deadfalls
*The game doesn’t count this as a proper “Rebel Camp” in the quest log, but it does have a Rebel Camp icon on the map. You complete this camp as part of Side Quest “The Bristlebacks“.
Rebel Outpost #2: Jagged Deeps
Rebel Outpost #3: Plainsong
Rebel Outpost #4: The High Turning
Rebel Outpost #5: The Dry Yearn
Rebel Outpost #6: Shining Wastes North
Rebel Outpost #7: Shining Wastes South
Rebel Outpost #8: Stillsands North
Rebel Outpost #9: Stillsands West
Rebel Outpost #10: Stillsands South
Rebel Outpost #11: Runner’s Wild
Rebel Outpost #12: Bonewhite Tear
Rebel Outpost #13: The Stand of the Sentinels
Rebel Outpost #14: The Graypeak
Rebel Outpost #15: Raintrace North
Rebel Outpost #16: Raintrace East
Rebel Outpost #17: Raintrace West
For everything else needed to 100% completion, check out the full Horizon Forbidden West Wiki & Strategy Guide.