Sable

I just want to add a few notes here.
1/ There's a toggle in the settings to make catching the fish easier.
2/ The achievement isn't unlocked when catching the last fish or when grabbing the last butterfly - but instead when delivering them to the Vivarium.
3/ After a few fish deliveries to the Vivarium (not the fish quest guy), I received updated rods, which greatly improved my chances for catching the rare specimens.
4/ There's very little written guides on the Internet (I don't like watching videos), but I found a great walkthrough on Steam that lists all of the locations including screenshots of the map, and some advice. It's great! The guide is by FelixDracul and is called "Sable – 100% Complete Achievement Guide (Includes Fishing Update)". Didn't want to do a direct link, since I'm not sure if that's allowed.