https://www.polygon.com/2018/11/29/18118588/pokemon-go-research-breakthrough-rewards-december

Pokémon Go players have another chance to receive two sets of popular legendaries in December, in case they missed out on them in the past. That is, if you’re good about doing your daily research tasks — as the legendary bird and legendary dog trios will be exclusive rewards for anyone who completes their seven-day research breakthrough streak.

The Pokémon Go Twitter account confirmed that December will bring the following Pokémon to gift boxes awarded to players who fill out their quest checklists for the entirety of December: Articuno, Zapdos, and Moltres (the birds), and Raikou, Entei and Suicune (the dogs). It sounds like which one you get is random, however, so all you can do is cross your fingers and hope you’ll get the exact legendary you’re looking for.