To celebrate 25 Years of Pokemon, a special Pokemon 25th Anniversary set of Pokemon TCG cards was released via McDonalds Happy Meals.
Each Happy Meal contains 1 Booster Pack, containing 4 different cards from the set. Each Booster Pack is guaranteed 1 Holofoil card.
Upon release, the set had issues with many people buying hundreds of Happy Meals, leaving many McDoanlds stores without any cards to give out. Many of the packs can be found on sites like Ebay.
The most sort-after card currently is the Holofoil Pikachu card.