The pint-sized primate was nabbed from his cage sometime after the Magnetic Hill Zoo in Moncton closed Tuesday.
Staff inspecting the squirrel monkey exhibit the following morning were "distraught" to discover that Hercules, the 19-year-old patriarch of the group, was missing, said zoo manager Bruce Dougan.
About four hours later, Dougan said he received a call from a man who claimed to know who had taken the tiny, black-and-olive coloured monkey.
"He was pretty sure he knew where Hercules was, and that some young men had stolen him as a university prank kind of thing," Dougan said in an interview Wednesday.
"They really didn't realize what they had done, the severity of their actions."
A couple of hours later, the man called back to say he had found Hercules and offered to return the primate — with conditions.
"He said he'd be willing to bring him to me if I was interested in getting the monkey back, and first and foremost not to involve the RCMP," said Dougan.
"So I agreed to meet him at a pre-determined location on a country road near the zoo. ... The monkey was in a cardboard box and appeared to be healthy."
Dougan said he didn't ask the middle-aged man too many questions out of fear he would get cold feet, but believed him to be genuine and not involved in Hercules's disappearance.
A spokesman for the RCMP was not immediately available for comment on whether police planned to pursue an investigation into the theft.
Whoever stole Hercules somehow broke into the zoo and cut through two padlocks to gain access to the outdoor cage, which houses five other squirrel monkeys.
Before the monkey was returned, Dougan said there was concern about Hercules's well-being because the small monkey, who weighs less than two pounds, requires a special diet that's rich in vitamins.
Dougan said squirrel monkeys are not very common in captivity and only live for about 21 years, which made the aging monkey's disappearance particularly distressing.
Hercules appeared to be doing well despite his ordeal, Dougan said, but he would be monitored overnight before joining his roommates on Thursday.
"He's very, very nervous," said Dougan. "It's very easy to see he's quite scared. I don't know what kind of trauma he's experienced ... but we're hoping that being in a quiet place and being able to settle down for 24 hours and then being reunited with his troupe will solve any of those issues."
Dougan said zoo visitors are often charmed by the mini monkeys.
"(He's) very, very cute," said Dougan. "And that's the appeal of squirrel monkeys, I guess, is that they appear so baby-like, so cute."
Staff members were also trying to figure out how the thieves snuck into the approximately 16-hectare zoo. Dougan said there was no obvious breach of the fence that surrounds the space.
Security staff, which includes a guard dog, didn't notice anything out of the ordinary between Tuesday night and early Wednesday morning, he said.
Dougan said the theft was the second in the zoo's history. Someone made off with a tortoise about 15 years ago, but the animal was never found.
In 2008, a baby Callimico monkey named April went missing from the Cherry Brook Zoo in Saint John, N.B., only to be found a couple of days later after police received an anonymous tip.
— By Melanie Patten in Halifax