The Anunnaki are a race of creatures living in another dimension. Every few thousand years, during an event called Nibiru, the barrier between their world and grow weak, allowing the Anunnaki to visit Earth.
The Anunnaki have no physical forms, so they have to inhabit various animals' bodies, thus being responsible for mythological figures as the Egyptian gods, China's Monkey King and the Aztec's Quetzalcoatl among others. It is revealed that the animals which can talk are descendants of the Anunnaki. Scooby Doo is one of them.
Those who're descendants have a longer lifespan than normal animals and have more intelligence like that of humans.
Scooby-Doo! Mystery Incorporated
An anunnaki possessed Nova while she was in a coma and spoke to Scooby-Doo in his dreams. She tells him about the time of Nibiru and how their race helped humanity over the years. But she warns him of the Evil Entity that's sealed below Crystal Cove and urges Scooby to destroy it and it's curse. (Stand and Deliver)
Nova reveals on how the Entity manipulated a group of four humans and an animal to set him free and instructs them to find the Heart of the Jaguar to destroy the Entity. (Nightmare in Red)
When the Entity was freed, he opened a portal from the sarcophagus and many anunnaki minions came forth and rounded up many townspeople for their master to feast upon. When Scooby destroyed the sarcophagus, all the minions and the Entity itself was sucked into the void and destroyed.
When the new timeline was created, Nova (still possessed) thanked Scooby saying he's the bravest dog of all as he and his friends left home. (Come Undone)
- Scooby-Doo! Mystery Incorporated
- Anunnaki is a real name for a group of gods in the Mesopotamian culture.