Fat snails are a dropped by Bruise blamish snails and Blood blamish snails in Mort Myre Swamp. They somewhat resemble Chad Buzhardt in body shape and weight. They can be cooked at level 22 to make fat chad meat, a food item, giving 95 experience. These snails can be used for the activity, Temple Trekking.
It may also be used in Fairytale I - Growing Pains as a quest item for magic secateurs.
If needed there is a snail that can be ranged or maged from the Digsite across the river. When the meat is dropped, you can use Telekinetic Grab (requires Magic level 33) to obtain the meat.