Symbiotes

The Klyntar (better known as the Symbiotes) are a race of parasitic alien beings featured in Ultimate Spider-Man, Spider-Man and Guardians of the Galaxy.

They attach themselves to their hosts and create a fused organism with a tendency for evil.

In Ultimate Spider-Man, a Symbiote is fused with Peter Parker's DNA. It attaches to Harry Osborn (creating the villain Venom) and then to Flash Thompson (creating the hero Agent Venom).

Doctor Octopus later genetically modifies the symbiote, creating a different symbiote known as Carnage. A shard of this symbiote splits off and fuses with Mary-Jane Watson (creating the heroine Spider-Woman).

In Guardians of the Galaxy, Symbiotes fuse wit the Guardians, creating "Venomized" versions of Star-Lord, Gamora, Drax the Destroyer, Rocket Racoon and Groot.

In Spíder-Man, a Symbiote fuses with Eddie Brock, creating Venom.

In Phineas and Ferb: Mission Marvel, Venom is featured, but it's unknown who is the host of the Symbiote.