The first Alien Symbiote was introduced in Secret Wars #8, when Spider-Man found what he thought was a neat new costume while battling in the Secret Wars.
After a while with the black costume, Spider-Man realized the suit was a parasitic alien that was trying to take him over. He rejected the symbiotic monster, which eventually found Eddie Brock. The two merged to become Venom.
Venom spawned several other symbiote "children;" Carnage, Phage, Riot, Lasher, Scream, and Agony.
Later, the Phage, Riot, Lasher, and Agony symbiotes merged into one being, known as Hybrid.
Carnage spawned the symbiote known as Toxin, who was the 100th symbiote in their lineage, and, as such, the most powerful.
Brock was abandoned by the Venom symbiote when it was discovered he had cancer. Eventually, it found its way to Mac Gargan.