#ishaku

BEETLES:

Issun's hat is the jewel beetle (Chrysochroa fulgidissima). A close contender is Acrothinium gaschkevitchii, a similarly-colored leaf beetle, but the jewel beetle's patterns and antennae fit more closely

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Ishaku's hat is one of the most obvious ones, being the iconic Japanese rhinoceros beetle (Allomyrina dichotoma). Specifically a male one since females have a much, much smaller horn

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The guards in front of Ishaku's house are wearing Japanese stag beetle helmets (most likely Prospocoilus inclinatus due to the shape of the mandibles)

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Hotaru is wearing a Genji firefly (Nipponoluciola cruciata) helmet. A close second is the Heike firefly (Aquatica lateralis), a very similar yet smaller firefly species. The colored spots on both N. cruciata and A. lateralis can be red, orange or yellow, but the more solid color pattern of N. cruciata fits better. Also the guy whose name is most commonly translated as "firefly" is wearing a firefly hat, haha funny

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Gengo's hat is... debatable. His original Japanese name is ゲンゴロウ gengorou, which can be translated as "diving beetle" or more specifically the golden edge diving beetle (Cybister chinensis), but I think his hat might be something more along the lines of C. sugillatus. C. chinensis has a bright golden edge all along the carapace that doesn't fit the hat at all, though he could have just cut it off or something? (The hats Ishaku and his guards wear are stripped of antennae entirely, so modifications aren't entirely out of the picture.) Other diving beetles like Hydroglyphus japonicus and Hydaticus pacificus conspersus have more obvious lighter spots, but they're a little too bright/stark to match the more subtler tones of the hat. In short: his helmet's probably one of the non-brightly patterned diving beetles, but I'm not sure which one it might exactly be so C. sugillatus is my best guess

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BUTTERFLIES:

Mura's headpiece is a great purple emperor butterfly (Sasakia charonda), also known as the Japanese emperor butterfly, which is the national butterfly of Japan. The white spots and purple color (which only the males have) are very distinctive

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Miya's headpiece is a swallowtail butterfly, most likely the Japan subspecies of the Old World swallowtail (Papilio machaon hippocrates) due to how bright yellow it is. The Asian swallowtail (P. xuthus) is a close second, though

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Mono's headpiece is difficult to place, but it's most likely the Japan subspecies of a small white butterfly (Pieris rapae crucivora)—the butterfly is known as "the small white", it's not just a descriptor. A lot of other whites in the Pieris family are fairly close as well, but either they aren't found in Japan or their patterns are too off. I'm thinking Mono just used two sets of forewings for her headpiece or something, because even the Indian cabbage white (P. canidia) doesn't have that exact hindwing pattern

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PLANTS:

Kunu's hat is an acorn from the sawtooth oak (Quercus acutissima). Q. acutissima acorns, as well as the acorns from a few other oaks native to Asia, are distinct from the usual western kinds due to their hairy cups. I had previously put the hat as a Japanese emperor oak (Q. dentata) acorn, but Kunu's original Japanese name, クヌギ kunugi, can be interpreted as 櫟, the kanji specifically meaning the sawtooth oak, so... Sawtooth oak it is

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Mrs. Seal's hat is obviously a pinecone that's probably from either the Japanese black pine (Pinus thunbergii) or red pine (P. densiflora). Both species produce very similar-looking pinecones and are probably interchangeable

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And that's all of them! Feel free to use this as a guide to what kinds of hats you can put on Poncles, or just as a fun little thing in general. I can imagine that most kinds of beetles, butterflies, moths and seed/nut shells are suitable as hats, if not anything similar to them