Genghis hated Tank with a passion, and the feeling was very much mutual.

The two dogs were, 95 percent of the time, sweet peas. Genghis was my 85-pound fawn boxer, floppy ears, docked tail. Tank was a brick shithouse of a Chesapeake Bay retriever. His owner was a burly, curly-haired guy with a big black beard who looked like he played upright bass in a band on the Americana charts.

Genghis would lick the bare toes of babies in strollers, and the babies would giggle maniacally. Tank loved to play with any human up for a game of tennis ball fetch.

And yet. If either of the two dogs spied each other from a distance, their barking would start. And escalate to Defcon 1 in a heartbeat. Theirs was not yippy barking, but rather a vicious cur display of hatred that communicated: “Get the fuck outta here, this is my street, move now or I’ll rip your fucking balls off you sonova bitch bastard…”

Upon sighting one another, Tank’s master and I would immediately cross the street and walk in opposite directions. We’d roll our eyes as each dog strained at their pinch collars, muscles bulging, eyes bugging, eager to end the other’s life or die trying.

Around this time fifteen years ago, Genghis’ life was in final descent, for big-breed dogs are not known for longevity. He left it all on the field after years of hard play during off-leash hours in Prospect Park. He had a luxated patella repaired (that is, knee surgery), and was riddled with arthritis.

Our walks grew shorter, our play times way less spirited. He’d chase his ball a few times, and then lie down on the grass, with the ball between his paws. He’d methodically peel and eat the fuzz off his tennis ball, as if it was the fur on a rabbit’s head.

My wife and I decided to leave Brooklyn after 25 years. There were a variety of factors. But we were out of there, and worried if Genghis would hang in there for the move. We’d leave for another round of viewings with realtors and return to find that Gengy had another horrific gastric accident behind a couch, or in a far corner where he thought we’d never find it. As if.

He was sick and failing fast, closing in on 12 years. We got him as a puppy from a home breeder in nowhere’s-ville New Jersey. He was a purebred Boxer but not a show dog. But he was our dog, our noble friend.

We breathed the same air for years, ever since I started my business and the little wildling would position himself under my bicycle, which leaned against a bookcase, and paw the pedals, his little white belly dotted with chain lube. I drove myself hard back then, building a portfolio of clients. Gengy would visit me around 11:45 or so, and jostle my right elbow with his massive head as I typed. It was as if to say, “Break time big guy. Plus, I gotta go pee.”

And then we’d go outside for our lunch hour constitutional. I’d be on the lookout for Tank. I’d feel vibrations run up Gengy’s leash as a low growl rumbled from my big dog’s chest. With Tank blocks away, I’d cross the street, or head into the park, but not before stopping at a stone fountain and filling a discarded Poland Spring water bottled plucked from a garbage can. I’d cap it and hand it to Genghis, who would proudly carry his beverage in his vise-like jaws.

Genghis never made it to the promised land. We had to help our furry friend, and a week before we moved, he stopped eating. It was game-over.

And so it was, too, for Tank, I was told. It was Ali and Joltin’ Joe Frazier. Two heavy weight champs, gone now for all time.

Their lives are so compressed. Their love is unconditional. And we, their humans, know the secret. That is, their clock is ticking.

As is ours.