Well, I was hiking around Hope County, a grapple trail near a lake at the Southwest end of Jacob's County, and happened upon a dead dude with a note next to him. The note states that an individual called "Bo" is interested in eagle feathers and fish bones, and judging by the location of the body, we can assume the man fell to his death, probably attempting to retrieve the eagle feathers. The note directly states that Bo wants the feathers from the top of the tallest peak, and the bones of a fish from the bottom of an unspecified lake. Finally, the note seems to promise some kind of extraordinary reward for our troubles, so I went ahead and followed the path, hopes high.

So after a lovely hike in the mountains, not even bothered by an encounter I had with a bear and a cougar caught in a stand-off, I eventually found this eagle's nest at the end of the trail.

Well, this obviously isn't the tallest peak in the game, however the tallest peaks seem to be unreachable by all means, so this is gonna have to do. No feathers appeared in the nest or in the area around it. No eagle spawned even after leaving and coming back several times, and conversely tossing all of my bait into the nest just to be sure, (to no avail, unfortunately...). The only thing I could interact with around the nest was a branch, which I picked up and threw into the nest a couple times, perhapse to simulate the action of an eagle building a nest? I dunno. Also tried punching, shooting, burning, and blowing it up, none of which had any effect whatsoever. Disappointing...

Nice view though...

With the eagles nest yielding underwhelming results, I decided to pursue the other clue. So I started by diving in the big unnamed lake near Joseph's compound, found a sunken house with a whiskey barrel in it, but no, "big old fish bones". Then I took a dip in the deepest areas of Silver Lake, the lake on the other side of the island, nothing there either. Perhapse I was hasty in going directly into the biggest lakes, and instead decided to take a look over at the lake nearby where I found the note. Probably should've tried that first. Oh well.

Once again, no fish bones. However, as seen in the above image, a number of piles of human remains lie in the deepest part of the lake, (3 or 4 piles if I recall correctly). Also, the skull of a longhorn which is near what looks like an underwater tunnel. Spoiler: it's not.
However, this to me seems like evidence that at some point in this lake there might have been a big fish, possibly prehistoric if "old" refers to age and not slang emphasis, chowing down on unsuspecting swimmers/fishermen, and livestock.

It appears as though there may be something more to this, if there wasn't I'd imagine the developers wouldn't have even bothered putting any of it in. Or maybe they put the nest at the top as a sort of finish line to the hiking trail, and the bones at the bottom of the lake for...fun or something? And they left the note just to lead people to finding both points of interest. Either way, it seems like there should be something more here, especially considering this unknown "Bo" character is promising, yet not delivering on that special something he said he'd craft for the individual to bring him the ingredients. Puzzling still, Bo left no way of finding him so maybe the person he sent was a close friend, or family member, and if we showed up with the ingredients expecting a reward, he may assume we were the cause of the fallen man's death. Well, anyway, I spent like an hour drowning and falling repeatedly, and out of it I got a fun hiking trip, did a little fishing, and made a dull guide dedicated to it. So all in all, even if this isn't really an easter egg, I had a good time exploring, and felt like sharing my experience.

Alrighty then, have a nice day.

So as it turns out, this area is a part of a side quest in which everything of relevance mentioned above is spawned in order to suit the story of the man with the note, effectively rendering this "guide" entirely useless. Oh well.

But look, I found the fish.

Thanks for reading.