I saw another dog being walked onto a plane with a leash but no carrier on BOS-ATL last night. It definitely was not trained as well as an actual support animal would be (it wasn't barking or being too horrible, but it did walk around and sniff things and have to be pulled by the leash during boarding). I would have been very annoyed if I had to sit next to it, but I was up front and the dog was somewhere way in the back. I never heard any complaints, and the plane took off on time, so I guess no one complained. I certainly would have though.