Game of Thrones featured a number of mystical creatures and non-human species over the course of eight seasons. Westeros' centuries of history is full of legends and ancient lore surrounding those beings that were thought to be extinct, including giants. So what's the origin story behind giants, and how many are left in Game of Thrones?
Giants were first mentioned in Game of Thrones season 1 when Tyrion Lannister visited Castle Black. He claimed that he didn't believe giants and White Walkers lived beyond the Wall. That same season, Osha shared her belief to Bran Stark that giants still existed. Many within Westeros thought that giants, dragons, and species like the Children of the Forest were simply the subjects of legends, but their existence was confirmed when Jon Snow traveled North of the Wall and witnessed a giant working alongside the wildlings. A few giants would later be seen during the Battle of Castle Black and the fight against Ramsay Bolton, and giants were also used by the Night King in his undead army.
Giants existed north of the Wall since the Children of the Forest first inhabited Westeros centuries before the events of Game of Thrones. Much of their history was forgotten, but there was thought to have been interaction with wildlings during their existence. The giants lacked the mental capacity of humans but they learned Old Tongue through the First Men. After the First Men pushed back the White Walkers, Bran the Builder enlisted giants to help him construct the Wall. The legend indicated that Bran might have used magic to enslave the giants in order to use their strength.
Throughout Game of Thrones, a number of memorable giants made their appearance. Mag Mar Tun Doh Weg, aka "Mag the Mighty," was the king of giants and a member of Mance Rayder's army. He was seen riding a mammoth at the start of the battle of Castle Black. He was killed during the battle, as was his fellow giant comrade, Dongo. The most notable giant was Wun Weg Wun Dar Wun, better remembered as "Wun Wun." The Free Folk giant was saved by Jon and bravely fought during the Battle of Castle Black and the Battle of the Bastards.
It's very possible that Wun Wun's death marked the extinction of giants, as there were no other known giants following Wun Wun in Game of Thrones. The only other giants featured were members of the undead army. Lyanna Mormont memorably stabbed an undead giant in the eye before succumbing to her injuries at the Battle of Winterfell. But as the lore suggests, just because more giants weren't shown, it doesn't mean that others don't still exist.
Since the Game of Thrones prequel is set to take place many years prior to the War of the Five Kings, it's plausible that giants will make an appearance. It's presumed that the Night King and the Children of the Forest will also have some sort of involvement in the prequel. The new series gives the creative team a chance at diving deeper into the fantasy that George R.R. Martin created with his novels.