The dog paid for its ticket
The dog obviously established a social media profile, took a photo of itself, and posted that they had paid for their ticket and were legal on the airplane. You’re telling us it didn’t happen like that? That’s settled that.
What about this? Someone cropped this photo originally posted on the internet and then harassed the person who took it? That does, after all, seem somewhat more probable. However, the poor dog wants to be somewhere but there.