The official Pokémon Tretta website has announced that starting in the middle of December 2012, Happy Sets purchased on the weekends at participating McDonalds locations in Japan will have Rookie Tretta pucks as the toy gift. There are three different Tretta pucks to collect: Kyurem, Pikachu and Kibago. Supplies are limited and the campaign will end when supplies run out.
Update: The website has updated with new info regarding the Rookie Tretta pucks that were distributed. December 15th and 16th Kyurem, Pikachu and Kibago were distributed. December 22nd, 23rd and 24th Eievui, Lucario and Sazandora were distributed.
Source:
http://www.pokemontretta.com/event/e121115_3/