Wait a minute, Pulse Wave. I don’t think that this pony is related to the Apple Family. But you were right. This pony does reassembled Sea Swirl/Seafoam. But she had a different cutie Mark and she was an earth pony. But since we didn’t get to see Sea Swirl in the movie, I would agree that she was present in the film attending the Friendship Festival in Canterlot, being arrested, enslaved, and/or siezed by the Storm King and his troops when he takes over and trashed Equestria into a kingdom of darkness and storms, and later present when Equestria was freed from the Storm King’s control, wrath, and/or tyranny over Equestria, the Storm King died, and the Friendship Festival resumes again.