Reshiram is now available within five-star Raid Battles in Pokemon Go, with both Zekrom and Kyurem following shortly. Aside from their obvious power, there's another reason you may want to stock-up on them.
The 'Tao trio' were first introduced in Pokemon Black and White — with each of them featuring as the box art for the Gen 5 games. Kyurem, while not featuring on the box, is able to fuse with either of Reshiram or Zekrom to become White Kyurem or Black Kyurem, respectively.
Interestingly the fused forms of Kyurem share the same Pokedex entry. This could mean you will need a Kyurem and Reshiram/Zekrom to get either of them.
How will White and Black Kyurem work?
Reshiram and Zekrom fusion material
We're yet to see this in Pokemon Go, but this could mark the first time two Pokemon are required to create another — this is why it is worth doing those extra Raid Battles just in case!
Besides, the worst case is they aren't needed and you end up with some extra super powerful legends... There really is little downside! In Pokemon Black and White 2, the fusion was reversible, but given the mechanics of Pokemon Go it's unlikely the mobile game will follow suit.
The Pokemon Go community also see this as a possibility, with one trainer saying: "It strikes me as entirely possible that PoGo may require you to use Reshiram/Zekrom as a 'fusion material' to make Black/White Kyurem."
Similar to Giratina
White Kyurem and Black Kyurem may work similarly to Altered and Origin Forme Giratina, where they are entirely separate Pokemon. They share the same Pokedex entry, but in terms of Pokemon Go, they have their own unique stats and moves.
If the same principle is used, White and Black Kyurem will fall under the Kyurem Pokedex entry. That would also mean you don't need to get rid of Reshiram or Zekrom to get one.
With the Therian forms of Tornadus, Thundurus and Landorus yet to be released in the mobile game, there are still a lot of questions about how they will be handled. How Niantic tackle this issue may reveal how Tao trio will be treated, too.
Reshiram will be available in five-star Raids until June 16 and will then be replaced by Zekrom until a yet-to-be announced date. Presumably it will be until July 7.