Almost everyone is familiar with the name Niantic now, even if they don't play the explosively popular Pokemon Go. John Hanke, CEO of the developer, was recently interviewed by the German edition of Wired. When asked when legendary Pokemon might arrive in the game he said, “I can say with certainty that we will see it more this year.”
Wired only mentioned five legendary Pokemon, specifically Mew, Mewtwo, and the three legendary birds Articuno, Zapdos, and Moltres. However, if Pokemon Go is going to include these rare Pokemon, it's safe to say that others will probably follow down the line.
Last summer, there was a big hubbub over a player who said they caught an Articuno, even though it wasn't available to catch. Niantic said that some players had received the Pokemon by accident. Seeing as how some players already got the bird on accident, it will hopefully mean the real release is soon to come.
Niantic takes Pokemon rares very seriously. “The value of Pokemon is that each one is very unique, and they have their own stories and characteristics,” said Tatsuo Nomura, Pokemon Go’s Director. Seeing as how legendary Pokemon are crazy hard to find in the actual Pokemon games, we'll probably see a similar effect in here.