The Snail is (as the name suggests) a large snail found in the Norn Terrarium, classified as a bug. If your snails have died off, you can use the Snail Agent or the Snail Launcher to replenish your stocks. It is a harmless herbivore and doesn't do much but roam about and eat plants. Occasionally, it will randomly hide in its shell.
Its image file includes sprites for vertical movement, but these are unused in the game.