pokemon-go
With the last update, Niantic added the Lucky Pokémons, which have unique sparkling background and reduced stardust cost when powering up. They can be obtained trading normal Pokémons, and the chances are higher the longer the time stored in your inventory.
https://pokemongolive.com/post/luckypokemon/
What are the real numbers?
Several communities confirmed those (including The Silph Road), Niantic didn't.
(*) South America is likely to have Tauros as well. This has been posted on Reddit several times (Tauros apparently got caught in Brazil), but not enough to confirm this as true. Feel free to update this once there's proof. Until then this stands as Tauros is for North America only.
Honestly I am simply translating your comment section into an answer and adding a bit. The algorithm is random (though I have no source) when it comes to Pokestops. Some times you will get 2 Pokeballs and 1 Potion, and other times you will get 1 Pokeball, 1 Greatball, an Egg, and a Potion. There is an unverified rumor going around [@Kevin L] about hitting your 10th unique Pokestop. They say every 10th unique Pokestop you spin you will receive a bonus - 50 extra experience and multiple extra items (still random). I hope this helps sum things up.
Best Answer
There are two ways to guarantee a lucky Pokémon:
You are guaranteed a lucky if you, or the trainer you are trading with, has acquired less 10 lucky Pokémon AND either Pokémon's capture date is July 2016 or August 2016 (Source)
With the new feature of lucky friends, trading while lucky friends will guarantee a lucky Pokémon. Lucky friendship status has a random chance to occur once per day when sending gifts, raiding, battling, etc with a best friend. (Source)
Outside the two situations where a lucky Pokémon is guaranteed, SilphRoad had done a lot of research on the chances. As shown on their site, below are the average rates for lucky Pokémon based on the capture date of the Pokémon