You’ve actually caught onto something that’s in the game. I don’t know if you’ve noticed, but on some trees across the map, there will be dreamcatchers hanging from them. These are actually collectibles of some sort. There are 20 locations across the map, and upon finding them all, it’ll direct you to the cave behind the waterfall at Elysian Pool, where a unique item will be there in that spot (it’s a bison by the way, not a cow). I don’t want to spoil any other details, but there are articles on the wiki you can read about it, you’ll just have to type in “dreamcatchers” and I’m sure they’ll come up. But you are indeed correct, that hole in the cave wall is there for a reason.