Most travelers assume that regularly scheduled airline passenger flights carry only regularly ticketed passengers, their luggage, an occasional small dog or cat and nothing more. You might be surprised to learn about who -- and what -- else may be flying along with you.

The next time you fly, take a good look around the cabin. There's a good chance the airline may be carrying some "special" passengers.

Recently a passenger boarded a Piedmont Airlines flight only to discover that he would be making the trip sitting next to a large, stuffed bull. The man who owned the thing found out that it would cost him more money to ship the giant stuffed animal by regular air freight than it would for him to simply buy it a ticket for a seat on the plane. He boarded the plane with the bull, buckled it into its seat and enjoyed the flight.

Last Christmas a very special passenger traveled from London with British Airways. "She sat in first class and was buckled in by the steward," says Deborah Bernstein, a spokesperson for the airline. "But she actually rode most of the flight on the flight deck with the captain."

When the plane landed in Boston, airport personnel whisked the VIP through customs and off to a local hospital, where the "passenger" -- a hard-to-find Cabbage Patch doll -- was presented to a critically ill young patient. (Unable to find one of the dolls in Boston, the child's family heard they were in stock at Harrod's in London and phoned British Airways for help. A British Airways' employee bought the doll at Harrod's.)

More often than not, many "special" airline passengers are alive.

There's Fifi, a koala bear who flew first class on Western Airlines in July from Los Angeles to Salt Lake City. Her kennel was strapped between the front bulkhead and the first row of seats in the 727.

Last November two 50-pound turkeys were flown by Piedmont Airlines from Raleigh Durham Airport to Washington National Airport. After landing, the birds, raised by the North Carolina State University Poultry Department, were presented to President Reagan by the National Turkey Federation.

Since l980, Republic Airlines and the University of Minnesota's Raptor Research and Rehabilitation Center have had a special flying arrangement. The center is a clinic for birds of prey -- some of which are becoming extinct -- that is affiliated with the university's college of veterinary medicine.

Whenever an injured or diseased bird is found, it can be taken to any Republic cargo check-in, placed in a special carrier the airline has available and flown to the university at no charge. A hawk, for example, was found on an off-shore oil rig. It had been injured when blown off course while migrating to Canada. The bird was transported to the center and, after its recovery, was flown to Canada -- also free of charge on Republic.

"We feel partially responsible for keeping many birds alive," says Walt Hellman, a spokesperson for the airline. "It has helped us make a lot of friends among environmentalists."

Last April, British Airways helped save the lives of three rare Kemps Ridley sea turtles. The turtles came into London with a consignment of tropical fish and a few days later were discovered in a pet store by customs officials. Since it is illegal to sell the endangered species, the turtles were returned to Tampa, Fla., on a jumbo jet. They traveled first class, of course, in a box lined with damp cloths.

British Airways also carried three 5-foot red-crowned cranes from Chicago to London and then on to their new home -- Moscow. The birds flew on a 747 in specially equipped plywood boxes with plexiglass windows.

Animals, however, don't always make the best traveling companions. There was the case of the macaw from the San Diego zoo, flying in a cage under a passenger seat. As macaw lovers know, the birds don't exactly chirp, "and this one," says Margie Craig, a spokesperson for Pacific Southwest Airlines, "screamed from take-off to landing."

Remember Penny the Penguin? A number of airline passengers do. PSA flew the Sea World bird in an unfortunately well-ventilated cage in the passenger cabin of a scheduled flight. Thankfully, like most PSA flights, it was a short one.

It is not uncommon for some passengers to buy a seat for a fragile item rather than risk shipping it as cargo. TV news crews usually buy expensive full-fare first-class seats for their camera gear. Professional musicians often travel with cellos or other large musical instruments strapped in to the seat next to them.

But not all airline "special travelers" ride in the passenger cabins. Almost every scheduled airline flight also carries special cargo. Most of the time you're probably unaware of what is flying with you.

On some Varig flights, passengers have flown while sitting on top of small submarines, sensitive oil drilling equipment and even bull semen.

Not surprisingly, Alaskan Airlines transports many dogs and dogsleds. (In fact, the airline has developed special plywood crates with separate compartments for each dog.) The airline also has the capability to reconfigure some of their planes according to how many passengers or how much cargo space they will need for a flight. The plane can hold 111 passengers and no cargo, 30,000 pounds of cargo and no passengers, 70 passengers and two containers of freight, or 50 passengers and three containers of freight.

Many times, the cargo is more important than passenger revenue, especially when the airline is hauling large shipments of Alaskan crab, salmon, halibut, scallops and rock cod.

British Airways recently transported approximately 500 of the historic works of art that are part of the Treasure Houses of Britain exhibit at the National Gallery of Art in Washington.

Pope John Paul II was the special cargo that Swissair transported from Zurich to Lucerne, Switzerland, last September. "We refitted the first-class part of the plane to hold a table and two chairs," says Wolfgang Schnekenburger, a spokesperson for Swissair. "We moved the bulkhead down a row." The rest of the plane's seating structure remained the same to accommodate the rest of the entourage traveling with the Pope. "We also put the Pope's insignia -- the papal coat-of-arms -- which is a bishop's hat with a ribbon, on the outside of the plane for him," says Schnekenburger.

Last year, Lufthansa flew l0 small, high-priced sports cars on a scheduled flight. They also flew a truck with earthquake detection equipment on it from Los Angeles via Germany to Chile, as well as a satellite to be launched by the space shuttle.

An Olympic champion flew in the cargo section of a Lufthansa Airlines plane -- the German gold medalist horse flew in a specially designed stall round trip from Germany to Los Angeles and back. "He even wore his gold medal around his neck on the flight," says Joe Zucker, a spokesperson for Lufthansa.

Those same stalls were then used to transport the Lipizzaner horses of the Spanish Riding School in Vienna as the animals made a special tour of the U.S.

But the cars, the satellite and the horses weren't flying below the passengers. They were riding behind them. Lufthansa is one of a number of airlines that fly Boeing 747/Combi aircraft. These are 747s that are three-quarters passengers and one-quarter cargo -- on the same deck of the plane. There is a bulkhead that separates the two sections. This cargo section has the same temperature and air pressure as the passenger cabins.

Every once in awhile, things can get out of hand between the cabins. There's the legendary story of the time passengers on a Delta Airlines flight found out that some passengers flying in the cargo section don't always stay there.

"We were loading these bees into the cargo hull," says Jim Ewing, a spokesperson for Delta Airlines, "and one of the packages opened up and the bees got loose throughout the plane -- hundreds of them. We had to evacuate the people and spray the plane to kill all of the bees." Delta still flies the shipments of bees, but insists the packages are now stronger.