The Horsehead Nebula was a nebula located within Mutter's Spiral.
During the Millennium War, the Mad Mind of Bophemeral attempted to create a wormhole in the Horsehead Nebula which would have killed quintillions. This attempt was stopped by the Osirians. (PDA: The Quantum Archangel)
The Uvodni and the Malakh fought the Ghost Wars against each other within the Nebula. (SJA: Warriors of Kudlak)
The Ood Sphere, homeworld of the Ood, is located here. (Doctor Who Annual 2007)