I am leaning toward believing the term was created by fans because of their "shiny" appearance but it could have been Nintendo trying to create game appeal to catch odd colored pokemon.
The term "Shiny Pokémon" is fan-made.
The term "Shiny Pokémon" was created by fans to refer to the sparkling sound effect and animation made at the start of an encounter with one in the games.
-Bulbapedia
Click here for more info on shiny pokemon.