As I am sure you know, Japan is a pretty safe place. If you have spent enough time travelling around Japan, chances are that you have been helped by a complete stranger, or been surprised at how considerate most people can be. Something about my face must give off the "I am clueless" vibe, because I have been asked many times if I need any help getting somewhere while waiting at the train station. I don't think I have ever felt safer than when I am in Japan.
Even deep in the mountains, Japan is safer than a lot of other countries. That being said, there are still some things that you will want to keep in mind while hiking or exploring the Japanese wilderness. Here are five dangerous critters that you should keep your distance from:
1. スズメバチ (Suzumebachi) or Japanese Giant Hornet
The suzumebachi, or Japanese giant hornet is probably the critter you have the highest chances of running into on this list. They are found all throughout Japan from Hokkaido to Kyushu. As you can probably tell from the picture, their size alone should be enough to make you keep your distance. If stung, they inject a venom that can be very dangerous, especially if you have allergies. There are a few different types of suzumebachi, but most are territorial, and won't go in for a sting unless they feel threatened. If you run into a one, don't make any sudden movements and back away slowly. If you are stung, it's best to visit a hospital as soon as possible just to be safe.
２．熊 (kuma) or bear
Japan has both black bears and brown bears (although brown bears are limited to Hokkaido). Bears tend to stay away from humans as much as they can and will normally flee if they hear any noise that gives away your presence. A lot of Japanese hikers wear "bear bells" to scare them off while hiking, but if you are with a group of people and continuously talking, bears should stay far away. Make sure you clean up camp if you are camping as any food left out could attract bears. If you do see a bear, stay facing the bear and slowly back away. Don't make sudden movements or try to quickly run away as they may attack.
If you happen to be attacked by the bear on the left, I have no advice for you. Best of luck.
3. マムシ (Mamushi) or Japanese pit viper (...but probably just mamushi)
The only creature on this list that I haven't crossed paths with myself (however, I just moved to a place where they appear from time to time). Found all throughout Japan, the mamushi hides in grasses and bushes waiting for mice, lizards, insects, and even some birds to chow down on. It is also the only animal on this list that is almost always waiting for its chance to ambush. They can be in bushes and trees above ground from time to time so keep your eyes peeled. They are venomous enough that you can be hospitalized for up to a week. In some rare cases, a mamushi bite may even lead to death. Japanese people generally try to stay out of any areas where mamushi might show up. Wearing good boots and keeping your skin covered can also help.
4. イノシシ (Inoshishi) or Japanese boar
The Japanese boar is mostly found in central/southern Japan. If you come across roots dug up by a wild animal, a boar is most likely the culprit. While obviously not poisonous, boars can run surprisingly fast (up to 50 km/hr), and have been known to injure people with their small tusks. Like bears, they will most likely keep their distance from you, but can be aggressive if they are startled by you, especially if they have their young ones with them. If you are attacked by one, back away slowly and don't startle them. Unlike bears, getting to higher ground is also a good idea as they will have no way to get to you.
5. 日本猿 (Nihonzaru) or Japanese monkey (macaque)
The bottom of the list brings us a creature that isn't life threatening, but can get still be dangerous. Also known as the snow monkey (or simply saru in Japanese), these monkeys are known across the world for being the "onsen monkies". They are generally pretty passive when in a good mood. I have been within a meter or two from them, and they really didn't seem to care. If you seem threatening however, or you egg them on, they can be very aggressive. They will expect a lot more food if you feed them even a tiny bit, so try to keep food out of sight. If you stare them down, or make sudden movements or loud noises when a monkey is close by, you could end up taking a couple blows to the face.
The guys at Gaki no Tsukai have played bizarre enough batsu games that they know what it is like. I don't want any piece of this rage:
There are other dangerous animals and insects that you may encounter in the forests or mountains of Japan, but being aware of your surroundings is the key. If you are careful and respectful to the nature that surrounds you, most animals will return the favor.
Stay safe and happy trails!
(Article Sources: personal experience; http://www.jon.gr.jp/qq/activity/sport/hiking/)