Here’s basically the facts we got from our conversation.
Nagini can’t be the snake Harry released because the snake in the zoo was a boa constrictor. Boa constrictors don’t have venom, and Nagini had highly poisonous venom.
Voldemort had to meet Nagini in Albania, as she was turned into a Horcrux following Bertha Jorkins’ murder.
Boa constrictors are brown, and Nagini is described as being green.
Voldemort also didn’t have any sense of a body before the beginning of the 4th book (when Wormtail rejoins him) so it seems highly unlikely that he had a snake prior to that point. It’s therefore likely that he met her immediately prior to Wormtail rejoining him so that Wormtail could milk her and create the potion that gave Voldemort the weird baby body.
I also speculated that Nagini was the final snake Voldemort possessed before Wormtail found him. She would have to be the last snake though because otherwise she would have died.
And it makes sense that she would be a Horcrux because Voldemort would be in a body of his own after he was done possessing her. So she would mean something to him.