Animal Crossing: New Horizons is going to see another change to its wildlife when August ends. There are a lot of critters leaving the Northern Hemisphere. I will tell you what you need to catch for your Museum and Critterpedia before they are gone. If you are in the Southern Hemisphere, then go here instead. First I will cover the bugs since they have the most critters leaving.
The Common Bluebottle and Great Purple Emperor are the two butterflies that will be migrating off your islands soon. The Common Bluebottle and Great Purple Emperor can be found from 4am to 7pm. You will have more trouble finding the Great Purple Emperor as they are not that common, but they are not rare either. If you keep catching butterflies, one is sure to spawn.
After those we have the Jewel Beetle which may give you some trouble. The Jewel Beetle can spawn at any time but it can only spawn on tree stumps. So make sure to cut down some trees to have a chance at catching this critter. If the Jewel Beetle is giving you too much trouble, just make sure to have a lot of tree stumps on your island.
Now for the long list: all the beetles that spawn on trees will be leaving. These are the Blue Weevil Beetle, Scarab Beetle, Drone Beetle, Saw Stag, Miyama Stag, Giant Stag, Cyclommatus Stag, Golden Stag, Giraffe Stag, Horned Dynastid, Horned Atlas, Horned Elephant, and Horned Hercules. All these beetles vary in rarity from common to extremely rare. The rare beetles are more difficult to catch, of course.
The best way to catch these beetles is by using mystery islands. Just make sure to bring a bunch of coconut trees to the mystery island since most of these beetles only spawn on coconut trees. I detail the entire process on how to catch these rare and elusive beetles in my beetle-catching guide. Feel free to farm them before they leave, as many of these beetles sell for 10,000-12,000 bells. You could make a fortune!
That covers all the bugs leaving at the end of August, now onto the fish. The Killifish, Frog, and Giant Snakehead can all be found in ponds and are leaving soon. The Killifish has the smallest shadow and can be found at any time so it will be an easy catch. As for the Frog, it can also be found at any time and has a size 2 shadow.
What will give you the most trouble is the Giant Snakehead as it is the rarest pond fish. The Giant Snakehead has a size 4 shadow so it can be confused with Catfish and Carps. The Giant Snakehead can be caught between 9am-4pm. Getting the Giant Snakehead to spawn is the tricky part. You can use fish food to try and get one to spawn.
Alternatively you may want to visit a Mystery Island and find one with a pond. Fish have a higher spawn rate on Mystery Islands so you can continually spawn away all the small fish while searching for the Giant Snakehead.
As for ocean fish, only the Napoleonfish and Squid are leaving. The Squid is quite common and can be found all day, making for an easy catch, but the Napoleonfish will be more of a challenge due to its rarity.
The Napoleonfish has a size 6 shadow and can only be found between 4am-9pm. I suggest using a Mystery Island when trying to catch this fish, because trying to find it on your own island can take hours. If you have any fish food, you can use it to try and get the Napoleonfish to spawn as well.
Now for the final category of critters, the deep sea creatures. Only two of these are leaving at the end of August: the Sea Pineapple and Vampire Squid. The Sea Pineapple has a small shadow, can be found at any time, and has slow short lunges.
What will be a challenge to catch is the Vampire Squid as it is rare and fast. It spawns between 4pm-9am and has a medium shadow. Due to its speed it will be difficult to catch and you will probably find yourself going across the entire ocean to grab it. Luckily, the deep sea critters do not de-spawn, so you can chase the Vampire Squid down until you catch it.
My best advice for catching the Vampire Squid is to swim up slowly to it. If it moves just continue to follow it without pressing the A button to continue sneaking up on it. The shadow will slow down, and once you are on top of it dive down and you will catch the Vampire Squid easily.
That covers all the critters leaving at the end of August from the Northern Hemisphere in Animal Crossing: New Horizons. The toughest ones to catch will be the rare beetles, Giant Snakehead, and Vampire Squid. But by utilizing mystery islands you will be able to catch them easily before August ends.
Good luck filling out your Critterpedia, and stay tuned for more Animal Crossing: New Horizons guides.