You should be especially cautious of the following wildlife
Bears: These are the most concerning, particularly in northern Japan, from Tohoku to Hokkaido. For more detailed precautions guide, visit: "The Bear Essentials: Your Ultimate Guide to Bears in Japan."
Snakes: Stay on marked trails to avoid encountering venomous pit vipers, which are commonly found in Japan’s forests.
Bees and Hornets: These insects are particularly aggressive towards dark colors, which they associate with their natural predator, bears. They are most active during the summer months.
Leeches: A major concern on wet days, as they can climb onto you from the ground or drop from trees. Some mountain ranges in Japan are notorious for leech infestations. Check your skin frequently.
Horsefly: Their bites can trigger severe allergic reactions. They are most active in the spring. Avoiding hiking in shorts during spring time.
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.