Pokémon Go looks to be receiving a new category of Pokémon. While initially ambiguous, there’s now a blog post with a full explanation about the new category of Pokémon available, called Lucky Pokémon.
Pokémon that have spent more time in storage are increasingly likely to be lucky. The Pokédex keeps track of them as well. When Pokémon are traded, there is also a chance that both traded Pokémon become Lucky Pokémon.
Not all of the information regarding Lucky Pokémon has been revealed yet, but information is bound to be released sooner or later. What we do know is that these Pokémon will require 50% more stardust to power up.