The Snalglí were a species of giant snail found on Vroengard. They were a popular foodstuff for the resident Dragon population. They were not inherently magical creatures, but were unaffected by most magic. They were over five-and-a-half-feet tall at maturity and the shells were believed to aid dragon digestion. They were eaten by Saphira in the chapter Snalglí for Two of Inheritance.
Like many creatures on Vroengard, Snalglí, were affected by the invisible poison that permeated the area. Exactly how is not fully detailed, but Glaedr indicates that their behaviour is unusual and two managed to pass, unaffected, through Eragon's Wards, while attacking him.
The concept of the Snalglí came from a friend whose reaction to escargot lead Christopher Paolini to create them.