I kept asking myself this very question while trying to catch the sea pig. For sure it is a real runner up but personally the barreleye fish. I honestly thought it didn’t even exist in the game because it seemed impossible that I couldn’t catch one even after 200 fish baits.
The barreleye was actually one of the first fish I caught but have not seen another since. I have tried with my alt on my main island and my island rep on my second island and can’t find one. The cicada shell was the hardest bug to find.
That darn golden stag beetle that I still haven't caught. I almost never see it, then I finally do and I'm too nervous and mess up the catch. Then season changes... one of these days I'm gonna time travel just to catch that thing....
I'm pretty sure the golden stag was the one that took me the longest to catch the first time. It took me a couple of hours on a mystery island where I was basically having everything but it spawn multiple times before I finally got one, lol.
Oh man it had to be those fast Sea Creatures I can't remember what it was but I think it was the "Lobster" that was swimming away from me really fast. It took me like 10 minutes to catch that thing.
I have an almost full insectopedia, I just need one more bug.
A scarab beetle.
Which, isn't in season for my N. Hemisphere island as it passed, and isn't in season for my S. Hemisphere island until January. I could just TT like I always do, but I'm tryin to keep a streak on my daily nook miles.
For my other pedias I have no idea. I don't fish alot (hate it) nor do I do alot of diving unless I want mermaid recipes.
Bug: It was the Horned Hurcules. I still can hardly ever find any even when I try, and when the time comes where I can catch one I possibly still will have hard times to catch one.
Fish: It has to go to the Mahi-Mahi. I mean, for real, going over so many Sea Bass just to find one, ONE! And when you do you should feel rewarded. I know I do all the times I have caught one.
Sea Creature: Well, I'll be honest with you, it was the Spiny Lobster that was the hardest for me. Every other one was simple to get, but that crustacean was the most stubborn one for me. Also, since I restarted I had to go through that again, thankfully it was easier to get than last time.
