A Symbol', also referred to as a Cutie Mark, is a marking, usually on the flank of a My Little Pony toy, that serves as a visual identifier. Symbols / Cutie Marks serve as a way to both identify individual ponies as well as to separate My Little Pony toys from other toy horses.
Flank markings first appeared in Generation 1, where they were known as symbols. The term "Cutie Mark" came into use during Generation 3, and has been used for subsequent generations. "Cutie Mark" is thought to be a play on the term "Beauty Mark". Generally, the symbol or Cutie Mark is a simple design, ranging from 1 to 3 colors, and represent objects or concepts. Though Symbols and Cutie Marks are usually simple in nature, some pony releases have featured more elaborate designs, such as Twice As Fancy Ponies, or sculpted saddle ponies like Merry Go Round Ponies.
In media, Symbols / Cutie Marks are usually seen on both sides of the body, however, the toyline alternatively, shown them on one or both sides, depending on the pose and packaging. Because the symbol is unique to a specific pony, it can be used as an identifier, though due to similarities in theme between symbols, it serves largely as a visual identifier rather than a descriptive one. Symbols are especially useful for Generation 1, where a single pony may have differences in body and mane color in different countries. Symbols and Cutie Marks usually relate to an overall theme for a pony, which includes the name and any personality traits used in marketing.
In Generations 1-3, the significance of symbols were not elaborated on. It appears that most ponies are born with a symbol, with the G1 pony Ember being a major exception. However, the mark often related to traits or hobbies that the pony exhibited.
However, Cutie Marks form a major part of the lore of Generation 4. In G4, ponies are born markless, but gain their Cutie Marks when they discover their unique talent. Cutie Marks are implied to make up a large portion of the magical powers of a pony, but are only rarely affected by outside magic. Cutie Marks cannot be forcibly granted to a markless pony by magic, however, a Cutie Mark can be removed by Tirek's magic stealing abilities or a rare spell that suppresses a pony's unique talents. Background ponies that appear in the show are given a cutie mark at random from a group of preexisiting marks. In general, Earth Ponies will have food or horseshoe based marks, Unicorns will have celestial based marks, and pegasi will have weather based marks.