The Female Snail is one of the 24 Golden Bugs found within The Legend of Zelda: Twilight Princess. It can be given to Agitha for a 50 Rupee prize. Once Agitha has received all 24 Golden Bugs, she will reward Link with the Giant Wallet, a massive wallet that can hold up to 1000 rupees.
The Female Snail can be found in the Sacred Grove. First, Link must go through the door to the south of the Sacred Grove into the past. Once he has, walk down the steps and take a right. Into the corner there will be the Female Snail up high. Take out the Gale Boomerang to capture it. The Male Snail can also be found in the Sacred Grove.