These are nasty sneaky snaky creatures born of Adro the evil Water Snake God.
They tend to hide in rivers and behind rocks, lying in wait for innocent humans. If you walk past one at night, watch out! The creepy creature will follow you home, licking its lips.
Whatever you do, don't turn around. If you so much as peep behind you, you've had it. Keep your eyes fixed ahead and pretend nothing is happening. That's the only way to stay safe.