Niantic has announced that Heatran will be in raids in Pokémon GO from August 21st, 2020 at 1:00 p.m. PDT until September 10th, 2020 at 1:00 p.m. PDT.

Battle Heatran in raids from August 21, 2020 at 1:00 p.m. PDT to September 10, 2020 at 1:00 p.m. PDT (GMT −7)

Trainers, The Fire- and Steel-type Lava Dome Pokémon, Heatran, will join Raid Battles across the world from August 21, 2020 at 1:00 p.m. PDT to September 10, 2020 at 1:00 p.m. PDT (GMT −7)!

Be sure to assemble a team that includes Water-, Fighting-, and Ground-type Pokémon to take down this Legendary Pokémon before it overpowers your team with its massive strength!

Don’t miss this chance to catch this Legendary Pokémon before it returns to its mountain home! Stay safe, and happy exploring!

Source: https://pokemongolive.com/en/post/legendary-heatran-2/