The Transportation Department issued a final rule Wednesday covering animals on airlines. It decided that only dogs can fly as service animals, and companions that passengers use for emotional support don’t count.

The rule aims to settle years of tension between airlines and passengers who bring their pets on board for free by saying they need them for emotional help. Under a longstanding department policy, all the passengers needed was a note from a health professional.

Airlines argued that passengers abused the situation to bring a menagerie of animals on board including cats, turtles, pot-bellied pigs and, in one case, a peacock.

The agency said Wednesday that it was rewriting the rules partly because passengers carrying unusual animals on board “eroded the public trust in legitimate service animals.” It also cited the increasing frequency of people “fraudulently representing their pets as service animals,” and a rise in misbehavior by emotional-support animals, ranging from peeing on the carpet to biting other passengers.

The new rule will force passengers with emotional-support animals to check them into the cargo hold — and pay a pet fee — or leave them at home. The agency estimated that airlines will gain up to $59.6 million a year in pet fees.

Airlines will be able to require owners to vouch for the dog’s health, behavior and training. Airlines can require people with a service dog to turn in paperwork up to 48 hours before a flight, but they can’t bar those travelers from checking in online like other passengers.

Airlines can require that service dogs to be leashed at all times, and they can bar dogs that show aggressive behavior. There have been incidents of emotional-support animals biting passengers.

Airlines for America, a trade group for the biggest U.S. carriers, said the new rule will protect passengers and airline employees while helping people travel with trained service dogs.

The Transportation Department stood by an earlier decision to prohibit airlines from banning entire dog breeds as service animals. That is a setback for Delta Air Lines, which banned “pit bull type dogs” in 2018, a move that was criticized by disability advocates.

Delta, however, is giving no indication of backing down. In a statement, a Delta spokeswoman said the airline is reviewing the new rule but, “At this time, there are no changes to Delta’s current service and support animal policies.”