Niantic is cleaning house in Pokemon Go by deleting Pokemon obtained by illicit means.
On Twitter, Niantic announced that it would be taking measures to make the game “fun and fair” by countering third party services that players can use to cheat. In this case, Pokemon caught using such services will be deleted and replaced by a red diagonal line, as a mark of shame in your Pokedex.
These Pokemon can’t be used, making them useless. One Twitter user helpfully shared what this looks like:
Source: IGN.com Niantic Deleting Illegal Pokemon Go Catches