The man goes by the name Will, and he spoke with a member of Case Breakers -- the team of more than 40 various investigators who believe Poste is responsible for the notorious murders in the late '60s -- about his relationship with Gary ... which he says started with Poste helping him with a legal issue.

We're told after that, in 1987, Poste invited Will -- who was just 18 at the time -- to come up to a wooded area in Northern California and hunt with him ... and he regularly did exactly that for the next 20 years.

Will claims during this time, he witnessed Poste kill animals "indiscriminately" and says Gary enjoyed "getting dirty" and bloody. He says Poste killed all sorts of animals and describes his behavior with their carcasses as "chilling."

It's interesting ... police profilers say an obsession with killing animals is a marker of a serial killer. The Zodiac killed his victims in the '70s and then stopped. If Case Breakers is right ... the killer may have turned to animals instead of humans beginning in the '80s.

We're told Will began to have suspicions about Poste being the Zodiac killer after watching a true-crime documentary about serial killers, and in 2014 he fled the woods and his life with Poste. He also felt the sketch of the Zodiac matched Poste.

Will then tried to talk to police and the media about Poste but says nobody would listen to him ... until he got in touch with Case Breakers about a year ago. BTW, Poste died in 2018.

As we told you ... despite what Case Breakers and Will believe, the official authorities -- the Riverside PD and a former San Francisco cop -- don't believe Poste is the infamous killer.

Whether that's true or not ... Will's account of hanging in the woods with a man who didn't have a conscience and slaughtered animals for joy is still incredibly disturbing.

We discussed Poste in this week's episode of The TMZ Podcast.