The direwolf population is dwindling in Westeros, but two of the litter have managed to survive. Arya's fierce fighter Nymeria is leading a pack, and Jon's companion Ghost is... somewhere. It's actually been a while since we've seen that big pup. Where is Ghost on Game of Thrones? I would guess Winterfell, but there has been little evidence in Season 7 to prove that.
In Season 6, Ghost was present when Jon Snow popped back up from being dead. He left Castle Black with Jon, Sansa, Davos, and Melisandre. He does not participate in the Battle of the Bastards, but does presumably move in to Winterfell when House Stark reclaims it.
He wasn't part of the entourage when the King in the North went to Dragonstone, was he? I can't imagine a direwolf would have sea legs. Game of Thrones writer and producer Byran Cogman tweeted that there was meant to be a scene between Jon Snow and Ghost in Episode 2, Stormborn. I assume that this was a goodbye before Jon boarded his ship to visit Daenerys — but it at least seems to confirm that Ghost is at Winterfell.
Unless he got on that boat too, I think it's safe to assume that Ghost has taken up in the kennels that Ramsay Bolton's hounds used to occupy.
FYI... We shot a Jon/Ghost scene. Didn't make it in. I tried! Thx for watching, everybody. #Stormborn— Bryan Cogman (@b_cogman) July 24, 2017
They've been separated before. Ghost and Jon spent some time apart while he was an undercover wildling and met Ygritte. Could the distance between them now be foreshadowing for his relationship with Daenerys? Every time he leaves his dog behind, he finds a new girlfriend. Something to think about.
All in all, I don't think Ghost's absence should be cause for concern, though it is a bummer that the direwolf likely won't meet a dragon for some time to come. The significance of the direwolves and the Starks is still a little bit of a mystery on Game of Thrones. Some, like Sansa and Bran, survived without theirs. Rickon and Robb did not have the same luck, and Arya and her direwolf have gone their separate ways.
Maybe Bran will warg into him now. Maybe Jon's massive pet can be the one to take out Littlefinger. Cersei's into poison murders now, and Sansa could make murder by dog her signature move. I'll be watching whatever happens to Ghost next with great interest.