Remember when spotting a dog in an airport terminal meant a red alert? Depending on your recreational habits, it could have been anything but a calming sight. Today, though, so-called therapy dogs are part of the passing parade at more than 30 airports—and counting—in the United States.

At San Francisco International Airport (SFO), for example, more than two dozen canines of varying sizes and breeds make up the Wag Brigade. The local chapter of the Society for the Prevention of Cruelty to Animals selects candidates for their temperament and “airport suitability.” The organization trains the canines in its Animal Assisted Therapy Program to invite passengers to pause, kneel and give them a few moments of doggy love. If the tail wagging isn’t enough, the vests reading “Pet Me!” are.

SFO launched its brigade in December 2013. The first airport to go to the dogs was Mineta San Jose International Airport (SJC), just after 9/11. A volunteer chaplain brought her certified therapy dog named Orion to the airport hoping he would calm travelers’ nerves about the prospect of getting back into an airplane. Now SJC, like SFO, has dogs that wag in the terminals seven days a week.

With positive feedback, the concept has kept growing. San Francisco lived up to its far-out reputation by adding a therapy pig. It’s the 50th anniversary of the Summer of Love, so why not give it up for LiLou, who is said to be hypoallergenic and knows a lot of tricks. According to an SFO website, she “can greet you with her snout or a wave, thank you with her shake, perform with her toy piano and bow at the end.” Apparently tech savvy, too, she even has an Instagram account.

Cincinnati/Northern Kentucky International Airport (CVG) uses therapeutic miniature horses. They’re named Denver and Ruby, and they visit from Seven Oaks Farms in Hamilton, Ohio, to stand still for nuzzles and pets twice a month. Previously, miniature horses with dyed manes were billed as “therapy unicorns” at this airport.

“It’s just to ease anxiety levels, put smiles on faces,” Wendi Orlando, airport customer service senior manager, told National Public Radio. “Clearly, that’s working. When you look at the passengers walking by, it just never gets old. They love seeing the horses.”