Just a side note none of these masks are needed to complete the game but they are an enjoyable little sidequest and fun to wear around Hyrule and get different reactions out people of the land! So please enjoy the guide and the masks!!
Keaton Mask: This mask is acquired after you show the guard at the gate of Death Mountain the letter from Zelda. He will tell you a sob story about how his son wants a mask that is popular and he heard of the Keaton Mask at the shop in Castle town but he is unable to leave his post to get it, he then asks of you to go and get the mask. Go back to Castle town and go to the Happy Mask Shop (I will shorten this to HMS) and purchase the mask and take it back to the guard. He will pay you what it was worth and you can take the money back to the HMS and pay for the cost of renting the mask.
Skull Mask: This mask is given to a lonely skull kid somewhere in the Lost Woods, wear the mask and talk to him and he will take the mask but however he will short change you and leave you to your own devices. So go back to the HMS and you're going to have to pay the difference out of your own pocket.
Hi Malon here, I just wanted to say that there is much more to be gained by this mask than is given credit for, for example go into the lost woods and in this order go, right, left, right, left and then left, you will find yourself in a room with two business schrubs and an old tree, walk toward the back of the room close to where a few butterflies are floating about (you may have to move around a bit to find it) but you should fall into a hole, inside is an empty room with a sign that says "Forest Stage, We are waiting to see your beautiful face! Win fabulous prizes!" where the skull mask on the stage and TA-DA! you can now carry twenty deku sticks, I know its not much of a prize but at least its a better reward for renting the mask than paying the shopkeeper.
The Spooky Mask: This is the mask you are allowed to borrow after you repay the Mask Salesman for the Skull Kid's mistake. This mask looks like the face of one of the many zombie like creatures that lurk around Hyrule. Now in order to get the next mask you have to go to the Kakariko Graveyard during the day and look for a little kid walking amongst the graves. He will tell you about how he wants to be Dampe the grave-keeper and says that his face is too cute for people to be scared. So wear the mask and he will buy it off of you. Now return to the HMS to pay the man back and borrow the next mask.
Bunny Hood: This mask's future owner is a man who runs about Hyrule field during the daylight hours, he is extremely hard to find during this time so wait until the night and he will have stopped to rest. After you find him he will tell you a story of how he wishes to be like the rabbits of the field. Wear the Bunny Hood and he will go berserk, begging and pleading for him to own it. Agree and he will give you a fortune of Rupees! Return to the HMS and pay back the salesman.
Mask of Truth: This is one of the four masks you can freely borrow after you have successfully found owners for the others. This mask will allow you to talk to Gossip Stones around Hyrule and learn information from them.
Gerudo Mask: This is one of the four masks you can freely borrow after you have successfully found owners for the others. This mask looks like one of the Gerudo thieves that live on the outskirts of the Haunted Wasteland. Wear it around to see what kind of reactions you can get out of it.
Goron Mask: One of the four masks you can freely borrow after you have successfully found owners for the others. This mask looks like one of the mountain dwelling creatures that take residence in Death Mountain. Wear it around to see what kind of reactions you can get out of it.
Zora Mask: This is one of the four masks you can freely borrow after you have successfully found owners for the others. It looks like the creatures that take residence in Zora's Domain near Lake Hylia. Wear it around to see what kind of reactions you can get out of it.