Warning: this list contains spoilers bigger than Drogon.
If there’s one thing Game of Thrones fans want to see more than anything else, it’s probably Jon Snow riding a dragon. However, while some theorize that Jon will ride one of Daenerys's dragons, others have a different idea: he'll end up on an ice dragon. There have been theories about an ice dragon in Game of Thrones for years, due to George R. R. Martin’s habit of peppering his writing with hints and foreshadowing. It all might amount to nothing, but then again, it all might amount to Jon Snow riding a gigantic ice dragon with freezing breath, and isn’t that the more awesome possibility to focus on?
Ice dragon fan theories mainly come from three sources: George R. R. Martin’s previous writing, in-text clues from the books in the Song of Ice and Fire series, and the wildest dreams of Game of Thrones fans everywhere. To see an ice dragon at this point in the story would be highly unexpected, but this is Game of Thrones, and unexpected is kind of what they do.
There’s more than one way to skin an ice dragon! A darker and more threatening theory on a potential ice dragon comes from Reddit user Paulnello. They think that the references to an ice dragon in the story may be foreshadowing a wight dragon. If one of Daenerys’s dragons is killed anywhere near the White Walkers and the army of the dead, there’s no reason to think the Night King can’t resurrect himself a zombie ice dragon. Perhaps the third dragon rider in the story will be the Night King.
If there is an ice dragon in Game of Thrones, and if Jon Snow is destined to be a dragon rider of some sort, it makes sense for Jon to ride the ice dragon. Of the potential dragon riders, Jon is the one most familiar with the cold, and he’s also the one who constantly invokes the ice dragon stories of his youth.
Reddit user i_am_the_grass posted a great breakdown of why Jon will ride an ice dragon, and although the post is a bit out of date, it gives solid evidence for the potential awe-inspiring team-up.
There is direct mention of ice dragons in the extended lore of Westeros. The World of Ice and Fire, the in-universe encyclopedia written by George R. R. Martin himself, says ice dragons existed in an arctic area known as the Shivering Sea. There aren't a lot of details, other than the fact that they could freeze things with their breath.
The information in The World of Ice and Fire is written by an in-universe maester, and thus the details are sometimes intentionally dubious. But GRRM specifically chose to mention ice dragons, and that might mean something.
Within A Song of Ice and Fire, the primary source of information on ice dragons is Old Nan. This beloved caretaker of the Stark children is known for enthralling them with her creepy, exposition-laden tales. A lot of what Old Nan talks about ends up being true, so book-reading fans have long since learned to trust in her words.
Her ice dragon stories made a big impression on Jon Snow, who often thinks of ice dragons in metaphorical terms, such as thinking that the wind was as cold as an ice dragon’s breath.