Aphrodite Clamar-Cohen, who teaches psychology at John Jay College in Manhattan and sees a psychotherapist, said her dog, a pit bull mix, helps fend off dark moods that began after her husband died eight years ago. She learned about psychological support pets from the Delta Society, a nonprofit group that aims to bring people and animals together, and got her dog, Alexander, last year. "When I travel I tell hotels up front that 'Alexander Dog Cohen' is coming and he is my emotional-needs dog," she said. She acknowledged that the dog is not trained as a service animal.

"He is necessary for my mental health," she said. "I would find myself at loose ends without him."

…These days people rely on a veritable Noah's Ark of support animals. Tami McLallen, a spokeswoman for American Airlines, said that although dogs are the most common service animals taken onto planes, the airline has had to accommodate monkeys, miniature horses, cats and even an emotional support duck. "Its owner dressed it up in clothes," she recalled.

There have also been at least two instances (on American and Delta) in which airlines have been presented with emotional support goats. Ms. McLallen said the airline flies service animals every day; all owners need to do is show up with a letter from a mental health professional and the animal can fly free in the cabin.