Niantic seems to make some changes on Pokemon Go particularly on some Pokemons' move sets. Based on the report, the developer has removed several moves and replaced them with new ones. Some Pokemon Go players might not know these changes but it actually happened a couple of months ago dated Aug 19, 2016.
According to the report from Otakukart, Pokemon that had already been captured before the said changes would not be affected. For instance, the Play Rough move set was removed and if you caught a Jigglypuff with Play Rough before the said removal, the move would stay as is. The Jigglypuff you owned that has a removed move set is now called as Legacy Pokemon. These Pokemon are now considered as rare nowadays since new Pokemon spawns contain a new set of moves.
Legacy Pokemon might have an important value in the future, so it is highly recommended to favorite them to avoid accidental transfers. In addition, these Pokemon might also have a greater trade value compared to other Pokemon of the same specie since it contains a rare move set. However, the release date of the Trading feature has yet to be announced as to date.
Meanwhile, here are the full list of Pokemon and their past moves; Jigglypuff's Play Rough, Diglett's Mud Shot, Dugtrio's Mud Shot, Graveler's Mud Shot, Golem's Mud Shot, Grimer's Acid, Muk's Acid, Gengar's Sludge Wave, Hitmonlee's Stomp, Chansey's Psybeam, Seel's Water Gun, Porygon's Quick Attack, Gyrados' Dragon Breath, Staryu's Quick Attack, Starmie's Quick Attack, Koffing's Acid, Wheezing's Acid and Omastar's Rock Throw.
It is highly recommended that players should definitely check their list of owned Pokemon to see if they have at least one of these Legacy Pokemon at hand. Moreover, inactive players and friends might also possess these kinds of treasures without knowing. If ever the Trading Feature will be introduced, it is highly expected that many dedicated collectors will look for trainers with Legacy Pokemon.