5th Anniversary Egg: Teeter Dance Pichu .

In the year 2003, various Pokémon were distributed as eggs with special moves, where in that particular generation or games, they normally have no access to those moves.

Additionally, Pichu eggs were distributed with a 5% shiny chance for the trainer directly receiving the event.
Anyhow, if the egg was traded to a different trainer before hatching, there is a chance of the resulting Pokémon hatched being shiny
(and consequently, a shiny egg could become non-shiny when such a trade occurs, as per any other egg).