Lucky Pokémon

Lucky Pokémon are special Pokémon that have minimum IVs of 12/12/12 and costs half as much Stardust to power up.

Obtaining Lucky Pokémon
When trading Pokémon, there is a chance that Pokémon can become a Lucky Pokémon, which is based on how long that Pokémon has been in the Trainer's box; the older it is, the higher the chance. (Those caught in July or August 2016 have the highest odds, able to reach 100%.)

There is a way to guarantee a Lucky Pokémon and that is by doing a trade with a Lucky Friend, which is a special upgrade to a Best Friend that has a chance of happening when you interact with them.

When traded, both Pokémon always either remain normal, or become lucky.

The Stardust cost for powering it up will be reduced by 50%, which means they can reach a higher CP much faster than other Pokémon.

Characteristics
On the Pokémon collection page Lucky Pokémon have gold glowing backgrounds behind their sprites.

In Pokémon summary screen, lucky Pokémon have a green label below their name, indicating that they are lucky. Their background is gold and features shimmering effects &mdash; this effect is also visible in their Pokédex entries which have navy blue background instead of purple. The number of each Lucky Pokémon a Trainer has obtained of one specific Pokémon is also tracked in the Pokédex.

Trivia

 * Although Pokémon turn into Lucky Pokémon at random when traded, the longer a Pokémon has been held onto, the greater the chances that they will become Lucky Pokémon.
 * Before September, 2018, Lucky Pokémon had a minimum IV floor of 10/10/10.
 * The sparkly effect of a Lucky Pokémon on the Pokédex actually extends slightly into the entries immediately before and after it.
 * The background of Shadow Pokémon completely override that of Lucky Pokémon on the same Pokémon. However, Shadow Lucky Pokémon can no longer be obtained as Shadow Pokémon were made untradeable.
 * The background from types used to be visible behind the Lucky Pokémon effect; this has since been changed and the type background cannot be seen on a Lucky Pokémon.
 * In September 6, 2018, Pokémon GO announced in Twitter that trading a Pokémon that was caught or hatched within July 2016-August 2016 will always guarantee a Lucky Pokémon.