A Kappa is the green bunny that swims around in the water.
You need to get a golden carrot, which spawn in either the carrot farm on the left side of the 'stage' or the carrot tree farm, which is on the right side of the stage. They will be floating in the air, press
to collect it, you can only carry one at a time.
Once collected, head to the bucket to the right of the carrot farm, it says, "DO NOT FEED KAPPA". Press
on the bucket and the achievement will unlock.