Venom is the final boss in Gradius V. It is the brain of Dr.Venom, which was assimilated into the Bacterian.
This hive mind unit doesn't attack, but due to the small space, you may ram into an eye.
It's possible that Venom could be born from Gofer because of the official story of Gradius V said that "Bacterion" was destroyed and his body broke into pieces in the past events of Gradius IV. Bacterion wasn't the final boss of Gradius IV but Gofer was. Then again, Gofer could be a part of Bacterion which means Venom could be born from Bacterion.
It's also possible that Dr. Venom was finally assimilated into the Bacterion Empire which could explain why there are multiple Zelos Forces at the end of the first level of Gradius V, and why some of the enemies that show up inside the Bacterian mothership look familar to Brain Golem.