Legendary rewards for Quests revealed for Pokémon GO

We have the first confirmation of other Legendary Pokémon besides Mew being obtainable through quests!

Pokemon Go Season of Heritage: Timed Research, Pokemon spawns and more - CNET

Master Pokémon GO Research

Pokémon Go Quests and Storylines – how do you catch Mew and what are the research tasks?

Guaranteed Deino & Meinfoo Encounters In Pokémon GO Battle League

Pokemon Go: Legendary Pokemon Dialga Headed to Raid Battles - IGN

Legendary Pokemon - Pokemon GO Guide - IGN

Pokémon Go Shadowy Skirmishes quest steps

Pokémon Go will soon include Zapdos as a research reward, might get raised level caps and more - Android Authority

Massively on the Go: Pokemon Go's Season 2 is Legendary