Game of Thrones season 7 trailer - shot-by-shot breakdown 'Enemies to the West'

Likely Euron Greyjoy, who currently hunts Theon and Yara, along with trying to take Danny as his wife. The ship looks surprisingly dragon-like. Also of note, Euron could hold the dragonbinder, a magical horn capable of subduing dragons. Perhaps Cersei and Euron will join forces - 'The enemy of my enemy is my friend'.