There are two unlockable monsters in this game. At some point, a boy named Taro will ask you to help him find a girl named Zoe. This little optional quest helps unlock the monsters.

Unlockable - How to Unlock

Boxdog - Simply help Taro & Zoe meet
Dogbox - Send Taro to goldsand dunes & Zoe to silverpeak gorge. Have the to meet in runedolf and they give you Dogbox, a better version of Boxdog