how do I get various misc items like Bone?

[Perryn ペリン]

Helicoprion, Leedsichthys, and Bone is all i'm missing

[Walde]

Helicoprion and Leedsichthys only appear when it's raining, while bone is just insanely rare junk item (at least in my experience, I've gotten more golden basses than bones!). You can use either the golden hook or the large lure when trying to get the first two, and a magnet lure for the bone to maximize your chances at getting them.

[Bonkers]

Bone is a rare trash catch, so use the magnet to hunt for it. The other two fish are rain-only, so hunt for rain or use the rain lure.