It's been nearly 30 years since I traveled with a pet on an airplane, but when I did, the airlines that allowed pets in cabin had specific limits (it was one or two pets per cabin total) and these had to be reserved in advance. If you showed up for a flight with an unreserved pet and the pet slots for that flight were already booked, your pet would not fly in cabin. So, if you are planning to fly with your pet in cabin this summer, best to book as early as possible.