Pokemon Go's latest Legendary, Heatran, made its debut in Raid Battles last month immediately following Cresselia's departure, but it won't be around for much longer. The Lava Dome Pokemon is scheduled to leave the mobile game on January 15, making this your last opportunity to catch one if you haven't yet.
As is the case with most other Legendaries in Go, to catch Heatran, you'll first need to travel to a Gym that's hosting a Heatran Raid, then team up with other players in-person to battle it. If you manage to successfully defeat the Legendary, you'll earn a handful of Premier Balls along with an opportunity to catch it.
Heatran is unique among all Legendaries thanks to its typing. The Lava Dome Pokemon is a Fire- and Steel-type, giving it a resistance to Dragon and Psychic attacks. However, this combination also makes it susceptible to Fighting- and especially Ground-type moves, so you'll want to bring along Pokemon such as Rhyperior and Groudon to battle it. Water Pokemon such as Swampert and Kyogre will also be very effective.
Heatran is the third Gen 4 Legendary to appear in Pokemon Go thus far, following Giratina (which arrived as part of Pokemon Go's most recent Halloween event) and the aforementioned Cresselia. Niantic hasn't announced what the game's next Legendary will be, although a new one typically arrives at the previous leaves, so we presumably won't have to wait long to find out.
In addition to Heatran, several older Legendaries are available in Pokemon Go once again through January's Field Research. You'll be able to encounter one of eight Legendaries--Articuno, Zapdos, Moltres, Raikou, Entei, Suicune, Ho-Oh, or Lugia--each time you achieve a Research Breakthrough through the end of February.
Pokemon Go's ongoing Adventure Sync Hatchathon is also scheduled to wrap up on January 15. Through the end of the event, you'll be able to receive twice the normal amount of Stardust and Candy for each Pokemon Egg you hatch, and you'll have a higher likelihood of receiving 5 and 10 km Eggs by spinning PokeStops and Gyms.