How do you get the dog whistle in terranigma

You find this in Norfest Forest. Make sure you have the Protective Bell equipped before heading in, or you'll end up infinitely looping back and forth between areas.

Eventually as you progress through the forest you will reach a dark area, and Ark will notice that he's being followed. It's in these sections of the forest that you have to look for a somewhat hidden path leading east through some tree stumps where you find the chest with the Dog Whistle. (You'll know you're in the right track when you can see water nearby).