“Another savage attack involving a pitbull, resulting in a close call with death! Story at 11:00!”
Considering the bias in the news lately, that’s about how I’d expect to hear the plug for this report — Rattlesnake Attacks Pitbull Near Bronx Zoo:

“Stone” is a 3-year-old Pit Bull with a swollen jowl and in very bad health. He was brought to a nearby hospital for emergency care after being bitten by a rattlesnake while out for a walk near the Bronx Zoo with his owner, Miguel Mota.
“He started going like this to his face,” Mota said demonstrating. “That’s when I knew it was something bad.”
Stone was sniffing around some brush in a park when he was bitten by the snake, putting the canine in immense pain.
“When I touched his face he cried,” Mota said. “I never heard him cry like this before.”
Mota walks Stone in the park every day and never saw a snake. And when Stone was bitten on Tuesday he never heard the rattling of his tail.
“I ran over to see what it was and I saw it slithering up this way to the bushes,” Mota said.
Mota went to three different veterinarian’s offices before he found Dr. Benjamin Davidson, and the anti-venom he desperately needed.
“He was displaying all the signs of rattlesnake toxicity,” Davidson said.
Stone was given several doses of the same medicine humans are given and that’s what kept him alive.
“Overall, the prognosis is probably good,” Davidson said. “The fact that he got the anti-venom so quickly is good.”
Mota was thankful for Dr. Davidson.
“The only hospital that had it,” Mota said. “Thank God or he would have been dead by now and I would have lost my baby.”
Some zoo visitors were surprised to hear of the attack.
“I can’t believe that can happen here, but we are near the zoo and animals sometimes escape,” Tenisha Gram of Queens said.
Stone will have to remain at the clinic for the next few days and be monitored in case of any complications.
Being bitten by a rattlesnake is rare, but the snakes are indigenous to the tri-state area.

I would think that if a rattlesnake had escaped from the zoo, the zoo would at least have known about it. Perhaps rattlesnakes are re-establishing their presence in the Bronx now that humans have stopped hunting them. (Rattlesnake hunts in New York probably went out of style in the 1890s…)
I think the most likely explanation is that this rattler was someone’s pet, and it got loose. Being indigenous to the area, it wouldn’t have had too much of a problem adapting, and there’s no shortage of rats, birds, and squirrels in local parks.
However, the incident reminds me of the Copperhead I found on a walk in Valley Forge National Park. Snakes are so well camouflaged that they’re all but invisible; even experienced snake hunters have a great deal of trouble finding them.
Unlike dogs, wild animals can’t be banned by bureaucrats. Nor do they respect city limits.