Howling Forest is a place where you can go on rescue missions. You can unlock Howling Forest by getting the friend area Sky Blue Plains (obtained by randomly doing any rescue missions that say the reward Friend Area). The next morning go to the Bulletin Board and a Howling Forest mission request will be there, sent by a Smeargle. Then access to Howling Forest is granted. After clearing it for the first time, you can also recruit the same Smeargle.