I'm finally removing all my Reddit posts/comments after years of no activity there, but I had one singular comment on the TotK subreddit that a lot of people found really helpful. I don't feel right erasing it completely, so I hope it's alright if I just copy over its contents here.

For a bit of context, around the release of Tears of the Kingdom there was a thread grappling with the problem of finding Boss Bokoblins of various colors. You need their horns for the Royal Gurad armor upgrades.

It can be a very frustrating problem to deal with because with enemy scaling, most Boss Bokoblins end up turning silver, but you still need the horns of lower tier Boss Bokoblins for the first few upgrades.

So around that time a couple of users, myself included, scoured the map for any Boss Bokoblins that are guaranteed to be of a particular color and never scale up to silver.

I had compiled some of those findings into one list, which is nowhere near complete, but that I felt had enough enemies of each color that you could reasonably get the materials you need.

With all that said, my comment back then was as follows:

I'm in a similar situation as you, most boss bokoblins I find are silver, but I haven't bothered to collect many of their lower level horns (I usually put them straight on my weapons whenever I encountered any).

I've been searching for them myself using the upgraded sensor and found four red bosses, which honestly should be enough between blood moons to gather enough of their horns:

• One is east of Lookout Landing, north of Mabe Village Ruins, which you've already found
• One is in the skull lair on the natural bridge west of Mount Gustaf, south of Carok Bridge and New Serenne Stable
• One is patrolling the Royal Ancient Lab Ruins, north of New Serenne Stable
• The last one is near the fork in the road north of the Coliseum Ruins, or directly west of the Exchange Ruins
• One more in the depths north of the Hyrule Field Chasm (found by /u/ViniVadaVocci)

I'll also need all the other colors of boss bokoblin horns, so I'll also be on the lookout for blue and black boss bokoblin locations (I don't think I'll have any trouble finding silvers).

On that note, I've found a black one already, in the skull lair just east of the Coliseum Ruins. Finding black bosses might not be that challenging, but I haven't seen any blue bosses yet.

Edit: I've been looking around a bunch, but I've only managed to find one blue and one black boss bokoblin, apart from confirming the other blues found by other redditors here.

For convenience's sake, here's my list of blue bosses, including the ones already mentioned by others:

• One within Crenel Peak Cave
• One southwest of Tal Tal Peak (found by u/metalredlink183)
• One directly north of the Spirit Temple (found by u/i_am_a_human1234)
• One on Outskirt Hill, just west of the Outskirt Stable
• One on Luto's Crossing Bridge (found by u/PM_Me_your_femboys)
• And another on Lake Darman on Death Mountain

And finally, my list of black boss bokoblins, including the one mentioned earlier:

• One east of t he Coliseum Ruins
• One in the Depths, southeast of the Floria Canyon Mine (Faron Region)
• On the Great Plateau, between the Temple of Time and the Eastern Abbey
• One west of Walnot Mountain (South Lanayru Region, found by /u/Minecraft_Obama)
• One in the depths underneath Nephra Hill (East Gerudo Region, also found by /u/Minecraft_Obama)

I'll try to update this list as I find more, but I don't think I'll be actively searching any longer, I've had enough of that. I hope I won't need to find any low level Captain Constructs either, I've noticed that those are also hard to come by.

Final Edit: Thanks for everyone's help. We've now got at least five entries for each color (other than silver, which you will eventually find everywhere), which I'm really happy with. Even if you don't have any boss horns whatsoever, just two blood moon cycles worth of farming this list will get you all the horns for the first three upgrades of the Royal Guard Armor.