There's no question that Daenerys has been bringing the fire to Game of Thrones Season 7, but you can't have GoT if no one brings the ice. The White Walkers, led by the Night King, are marching on Westeros and have an army of the dead behind them. Undead soldiers and giants are impressive, but they may not be a match for Daenerys' fire-breathing dragons. Unless, of course, the Night King has a dragon of his own on Game of Thrones.
The Night King is already powerful as it is, but a Dragon beneath him would make his army nearly unstoppable. After seeing how Daenerys' army absolutely devastated the Lannister army with Drogon, what could the Night King do with his, if he had one? Would three of Danerys' dragons be able to take on one of the Night King's dragons? Are "ice dragons" even real? If the Night King has an ice dragon, why didn't he use it from the very beginning? Evidence suggests that the Night King doesn't have an ice dragon, but that doesn't mean he couldn't ever obtain one.
While they've never been featured in the show, ice dragons are real in the world of Game of Thrones. They're ancient and mysterious beasts that many people believe to be fictional. However, people also thought regular dragons were gone before Daenerys showed everyone that they were very real. However, it looks like the Night King isn't in possession of one (we'd likely have seen it already if so), so these creatures may truly be extinct. The Night King probably won't be able to find and train an ice dragon in time for his war on Westeros — but he could make one.
The Night King has already proven his ability to turn humans, horses, and giants into wights. But what happens if he attempts to turn a dragon into a wight? Would an undead dragon still breathe fire? There are only three known dragons in the world, an if The Night King managed to kill just one of Daenerys' dragons and turn it, he would become the most powerful force in the world. If the Night King managed to turn two or even all three of Dany's dragons, then it is officially game over for Westeros.
The Night King doesn't need dragons to have a fighting chance at taking Westeros, but having something to battle Dany's dragons would make things much more even. The fight between the dead and the living already promises to be the most massively brutal and intense fight in Game of Thrones history. But if dragon-on-dragon combat becomes a part of the battle, then this could easily go down as the greatest Game of Thrones battle scene of all time. The Night King may not have an ice dragon yet, but if he's going to win the war on the living, then he needs to figure out a plan to get a dragon of any kind on his side.