The storyline described for the acquisition of the Venom symbiote varies.
In the comics, Peter got it during the Secret Wars, when he accidentally used the wrong machine in their high-tech base to create a new costume after his original had been badly damaged in combat. What he ended up doing is releasing the symbiote. He thought it was just some cool new technology that adapted itself to whatever he wanted, but eventually he learned the truth, rejected it, and it joined with Brock against him.
I believe the cartoon did the story about it being attached to a returning spaceship. If they do put Venom or Carnage in future movies, I imagine it'll be something like this, because there's no way they can pull off a Secret Wars movie.
...unless the rumors of a Pixar/Marvel collaboration are true
