Great White Whisker is a trophy item and you can get it from the carp sub quest.
We have listed down the detailed method to get the Great White Whisker.
How to get Great White Whisker:
Go to the Pot noble in the Hirata Estate and purchase the required items.
Make your way to another Pot noble who is located at the Fountainhead palace. Go through a long cave to reach the Pot noble. Buy the required items and they will give you Truly Precious Bait.
Now you need to feed the Great Colored Carp with this bait. Follow the same path to go back and ring the bell on the Fountainhead Palace and throw the bait on the Great Colored Carp to eat.
Talk to a guy near the bell and then feed the Great Colored Carp again with the Precious bait. And then go to the Guardian Ape’s Watering Hole.
Look for the corpse of the Great Colored Carp and Great white whisker in the Sunken Valley.