Originally Posted by
kipper
There's a difference between a service dog and an ESA. Also, any dog can have times where their behavior is off. An ESA could growl or snarl at another animal it sees, if it feels the other animal is a threat (like if the other dog is close to attacking the ESA's human). Growling and snarling is one way dogs communicate. If they feel another dog or even a person is a threat, they may growl to communicate that. Empress Cobaka, who isn't an ESA or a service dog, but who loves all people, growled once, when she felt another person was a threat towards me. As a side note, I would much rather have a dog growl than not, because a growl tells me a dog is unhappy before they bite. A dog that has been trained not to growl may simply attack without audible warning.
Of course most dogs (including genuine service dogs) would react if their owner is threatened (that's why my post said "for no reason"), but they can and should be trained not to growl at some other animal across the room that's not interacting with them. ESAs don't perform any particular task, but they can and should be trained to behave in the situations into which they're being taken, including airports and airplanes. In fact, it would not be unreasonable to expect a true ESA to be trained along the lines of the "certified" animals that officially visit hospitals, nursing homes, airports, etc. to provide emotional support.