A Poogie can be found on the bottom floor (Tradeyard) in Astera. After each quest, visit the Poogie and successfully pet it when it gets an exclamation mark over it's head (you only need to do this once per quest). After a certain amount of quests, the Poogie will get really ecstatic when you pet it and jump really high. This will now allow the Poogie to find new costumes and items for you. Note that may still need to do one more quest after getting the Poogie to be this way. Pick up the Poogie and bring it to the designated locations (your controller will vibrate when at the correct location) and put it down for it to dig up an item. Then take the item it dug up to permanently unlock the costume. You need to do a quest each time to get another item.

Unlockable	How to unlock
Apprentice Fiver	Tradeyard, by the Commander's area, next to the white flag.
Emperor's New Duds	(Requires story to be completed) The highest location you can go in Astera, in the scouting tower.
Hog in a Frog	Tradeyard, at the Ancient Tree by the torch on the right.
White Jammies	Tradeyard, by the fossil in the corner by the Smart Biologist.