The new pokemon have much more abstract/eccentric sprite designs. Previously, I thought of the new designs as a message from nintendo saying "help us, we're out of ideas". But I have changed my views and I now find them okay overall.
And yes! I guess that the third legend in the mascot trio, including Reshiram and Zekrom, would be National Dex #646: Kyuremu. Click on the image for info.
Kyuremu is a Dragon/Ice type and has Pressure as its ability. However, it isn't much like Giratina or Rayquaza. This is because its counterparts Reshiram and Zekrom have signature abilities. Also, Kyuremu's signature STAB move has a measly attack damage of 65, compared to Reshiram's massive 150 fire-type move and Zekrom's 130 electric-type move. Furthermore, both of the mentioned moves are learnt only at Lv. 100 (like Judgement,Seed Flare,Magma Storm and Crush Grip).
But do not forget, it has only been days since the release and everything that I have typed here might be inaccurate. So, I'll update on Kyuremu as soon as I know more. *all reference for this post is from Psypokes.