Granny Smith is an elderly Earth pony. She is Applejack, Apple Bloom, and Big Macintosh's grandmother, and a recurring character in the series. Like nearly all of the Apple family's cutie marks, hers is an apple based product, in this case a pie.
Depiction in the series
Granny Smith, named and colored after the apple cultivar, first appears in Friendship is Magic, part 1 as the last member of the Apple family whom Applejack introduces to Twilight Sparkle. She wakes up and gets out of her rocking chair, mumbles a few words like "what?" and "soup's on", then feebly walks over to join the rest of the ponies. She mutters "soup's on" again in The Show Stoppers when Scootaloo passes by her. She speaks more coherently in Griffon the Brush Off, when she mistakes Gilda's tail for a rattlesnake, and from the second season episode Luna Eclipsed onwards.