Dogs are known as a man's best friend, but not for one Delta Air Lines passenger who says he got booted from his first-class seat by a service animal.

After his annoying experience, the passenger posted a pic of the furry pooch on Reddit and ranted about what happened to him.

Under the user name @ben_bob, he told others in a forum he was upgraded to first class before his flight only to be downgraded 15 minutes later to a "worst seat than I previously had."

He took up the issue with a desk agent who only said, "something changed."

Moments later, the man saw the dog sitting on the floor in front of the empty bulkhead seat with extra legroom he was momentarily assigned to. The canine's master was seated next to the mutt holding it by a leash.

That threw the passenger over the edge, as he fumed on Reddit, "Okay, fine, I am disgruntled but whatever, I then board only to see this dog in my first class seat. And now I'm livid."

He was so upset that he contacted Delta, and an agent told him people "may be relocated for service animals" and "there is nothing they can do."

However, per Delta guidelines, service dogs are to fit within the foot space of passenger seats and not intrude into the aisles or space of other customers.

The angry passenger also complained in the Reddit forum, "There is no way that dog has spent as much with this airline as I have. What an absolute joke."

He added that it's now pointless to be a loyal customer to Delta and is questioning his allegiance to the airline because he and other flyers feel mistreated.