I have to disagree. He's most likely not a warg, or the much rarer greenseer. And Brynden Rivers is called 'the Last Greenseer', also I don't see him teaching someone like Euron. Everything we've seen, so far, from Euron is eastern magic. IIRC, he was crazy enough to sail to the ruins of old Valyria, which then made him crazier.
The thing is that Bloodraven most likely taught Euron before he knew what he was and once he realized that he discontinued his lessons.
If Brynden Rivers is the last greenseer then Bran isn't a greenseer or else he isn't the last. Anyways all three of them are greenseers. It's just that Brynden has realized his potential and Euron and Bran haven't.
Bran is interested in the Children of the Forest and the legends Beyond the Wall and Euron is interested in Valyrianand is trying to position himself as Valyria's heir even trying to get married to Daenerys and ride a dragon.
Euron's plotline is also converging with the white walkers and Daenerys'.
The passage on the Bloodstone Emperor in AWOIAF is foreshadowing of what Euron is going to be doing:
"His was a reign of terror. The Bloodstone Emperor not only practiced torture, but also dark arts, necromancy. He enslaved his own people, took a tiger-woman for wife, feasted on human flesh and cast down the true gods of Yi Ti, to worship a black stone that fell from the sky. The Blood Betrayal, as his usurpation is known is the annals of the Further East, ushered in the Long Night, in which the Maiden-Made-of-Light turned her back upon the world and the Lion of Night came forth in all his power to punish the wickedness of man.
He's believed by some scholars to have beeen the first High Priest of the sinister Church of Starry Wisdom."
Anyways like I said there is too much evidence of Euron and a at being connected that doesn't make sense unless Euron is a greenseer which BR would be interested in.
