Mamushi
I spotted a mamushi (マムシ) while hiking in the mountains today. This species of snake is venomous, and it is the most dangerous snake in the area that I live. The one I saw today was about sixty centimeters long. It was already on the move when I spotted it, and I lost sight of it quickly.
A bit further down the trail, I ran across a forest rat snake that was quite a bit larger than the mamushi (but nowhere near as dangerous)!