The 2006 horror flick Slither is an excellent mash-up of VHS horror influences and it relishes the monstrous, parasitic lifestyle of its central alien menace.
I’m not gong to lay out the life cycle of the Long One as I think the Alien Species Wiki does a pretty fine job of it. But what you have here is your typical biomass-consuming world breaker, with certain similarities to terrestrial slugs and snails. In its primary form, the organism infects its primary host via a needle or dart—perhaps inspired by the “love dart” used by some slug and snail species to flood hormones into a mate. And when the primary decides to reproduce, it uses a pair of tentacle-like organs to impregnate a host.