Originally Posted by Phoenician Aviator
However, my question is much more related to whether the airline would refuse to let the dog occupy the seat paid for with a ticket on the grounds that the passenger in question is a dog.
One would need to consult the Conditions of Carriage for the airline to get answers to such questions. Here are some standard definitions:
Passenger means any person, except members of the crew, carried or to be carried in an aircraft with the consent of carrier.
Passenger Ticket means those portions of the ticket issued by the carrier that provide for the carriage of the passenger.
Since an animal cannot be considered a "person", it, consequently, cannot be considered a "passenger".