The ancient supernatural White Walkers are the main antagonists in Game of Thrones, but they won't pose a significant danger in House of the Dragon.
Game of Thrones has already indicated how many of the prequel's worries will pan out, just as many twists in the wider plot of House of the Dragon.
Along with showing that the Targaryen dynasty will end more than a century after House of the Dragon, Game of Thrones also makes it clear that when Arya kills the Night King, the mythical White Walkers won't pose a danger to the kingdom any longer.
Over 8,000 years before the events of House of the Dragon and Game of Thrones, the Children of the Forest initially developed the White Walkers as a means of combating the First Men.
The White Walkers split from the Children and influenced The Long Night, the longest winter in recorded history, when they invaded Westeros and attacked the living. In the end, Westeros prevailed and drove the White Walkers far into the North, where Westeros then built the Wall to keep them out.
The White Walkers made a comeback in 298 AC after spending a long time concealed beyond the Wall. This culminated in the Battle of Winterfell, where the Night King, White Walkers, and wights were all vanquished.
Considering it’s still another 150-200 years before the White Walkers officially return, House of the Dragon will not feature the ancient magical creatures as villains or characters.
The White Walkers continued to be mythical beings whose tale, whether believed in or not, would be passed down for decades, therefore it's likely that they will still be referenced in passing throughout House of the Dragon.
The White Walkers and the Night King may instead only be utilised as the protagonists in spooky tales given to kids, as Bran Stark's eerie recollection from Old Nan in season one of Game of Thrones.