The Male Snail is one of the 24 Golden Bugs found in The Legend of Zelda: Twilight Princess. It can be given to Agitha for a rupee prize. Once all 24 Golden Bugs have been given to Agitha, she will reward Link with the Giant Wallet, a massive wallet that can hold up to 1000 rupees.
The Mail Snail can be found in the Sacred Grove. From the Pedestal of Time, travel toward the destroyed staircase. Take a right and walk into the cave. Stop a little bit in, and turn around. On the wall Link will see the Mail Snail. It can bought down using the Gale Boomerang or the Clawshot. The Female Snail is also found in the Sacred Grove.