A forest dweller tried to defeat the rhinobores that were threatening his home all by himself.
This errand is available to complete after defeating Hickory Dock in Ding Dong Well.
To begin this errand, you must first find this mysterious forest. Take a look at your map; directly to the east of Ding Dong Dell is a path between two mountain ranges, with a forest right in the middle of that path. Just to the north of THAT forest is the one we need to visit, so head east to the smaller forest, then north of it to arrive inside.
Once you're in the forest, talk to the boy beside the chest. He'll tell you that five Rhinobores are destroying their homes, so you'll be the one to slay the beasts.
Back out on the World Map, you'll see all five of them surrounding various parts of the forest. They are actually pretty easy; just go into battle and let your Thumbelemur have a sparring session with each one to quickly knock them out.
Return and talk to the forest boy and you'll complete the errand.