List of unidentified species

During its history, the Milky Way galaxy has played host to many intelligent forms of alien life. While many can be grouped under the wider banners of the Forerunner Ecumene, the Covenant empire or the auspices of the Unified Earth Government for humanity, many alien encounters have been recorded with no currently-known species responsible.

Forerunner indexing
According to a Catalog, before the firing of the Halo Array on 97,445 BCE, the Librarian studied and favored at least 123 species across three million worlds in regions the Forerunners explored in the Milky Way galaxy. The Catalog was uncertain however how many the Librarian aimed to preserve. While many sentient species were ultimately indexed and reintroduced, many of them did not reach Tier 7 of the Technological Achievement Tiers after the firing of the Halo array, where only a handful were encountered by Humanity in the Orion Arm of the galaxy, and not every species that were in the Covenant were cataloged by the United Nations Space Command, with even the Covenant client species keeping few records of their affairs with them.

Great dense cloud
Following the firing of the Halo Array, a final communication was heard by 343 Guilty Spark at Installation 04 from a "great dense cloud". This was sent from a species that just managed to send out their first communications out from their home system. This species was not encountered by the Forerunners or Flood; and ultimately was not indexed by the Librarian for reintroduction. Guilty Spark did not understand their message, but presumed it was hopeful signals. After the Halo's pulse hit the cloud, the communication stopped.

Installation 04 (Alpha Halo)
Over its operational lifespan, several unidentified visitors have found their way to Halo Installation 04.

Crashed ship
Around 40,000 BCE (50,000 years following the firing of the array), an Unidentified alien vessel crashed on the ring during an experiment 343 Guilty Spark conducted on the Halo. None of the ship's occupants attempted to leave the vessel's crash site or attempt contact, leading Guilty Spark to build a sarcophagus around the vessel. Ultimately, the Monitor was hopeful for the next species to arrive on the ring.

Visitors to the ring
During the Battle of Installation 04, 343 Guilty Spark noted in logs made during the battle that the Covenant landing on the ring and seeking something was similar to another alien species who previously arrived at the ring, recorded what they saw and left. These visits ultimately lead to 343 Guilty Spark changing his excitement at the prospect of meeting an alien species to defensive; threatening ships that trespassed near the ring including the in 2552.

Installation 05 (Delta Halo)
It has been suggested that outside meddling may have caused the breach which lead to a Flood outbreak at the installation's containment facility, culminating in the outbreak and eventual formation of a Gravemind on Installation 05.

System of Miasmic Giants
Around 851 BCE, the Sangheili 'Crecka recalled a event that happened "many cycles ago" in the System of Miasmic Giants where a hostile species attacked a vessel he served on. During the attack, 'Crecka was the last survivor and managed to escape by piloting the ship randomly though slipspace. According to 'Crecka, the species did not speak a civilized tongue.

The Refuge
According to Bal'Tol 'Xellus, an alien prisoner from a far off planet found The Refuge, where it taught 'Xellus' ancestors a meditation that would be passed on for generations until it was passed onto them.

Line Installation 9-12
Among the debris found at Line Installation 9-12 includes a number of ships belonging to species not known to be Human or Covenant client species.

Netherop
At some point in its distant past, the planet of Netherop hosted a civilisation and an intelligent species. The species was assumed extinct by John-117.
 * In The Military Survey of Uninhabited Planets, the Office of Naval Intelligence had a classified addendum that said Netherop was possibly abandoned - implying that the original surveys of the planet may have found artificially constructed terrain features. In 2559, while working with the Military Survey of Uninhabited Planets, xenoarchaelogist Keely Iyuska discovered a record stating that at some point before the civilisation's disappearance, the Forerunners had dispatched a Guardian Custode to subdue Netherop's population. Much to Iyuska's surprise, the civilisation had managed to destroy the assaulting Guardian.
 * Forgotten Highway: A stone highway that was not made by humanity was present on the planet. It was cut into the cliff on what humanity called the Serpentine Canyon.
 * Mountain runners: Oval-bodied, ten-legged spider-like machines made by an unidentified species were used by the Castoffs on the planet. These vehicles were not made with ferrous metal, meaning a leg could not be attached to Frederic-104s magnetic mount. The machines had a driver and passenger compartment and control of the vehicle was done with a control orb. The machine was slightly bigger than an M12 Warthog. The machines contained a coal-fired machine that charged batteries.
 * Microwave-beam weapons: The Castoffs on Netherop made use of weapons that looked like a sickle-shaped dish antenna and fired a microwave burst. These weapons were mounted on the mountain runners.
 * Cave Cities: The unidentified species created cave cities, before Lena and her ancestors were marooned on Netherop. These were abandoned and had no food when investigated by the humans on Netherop, but they did contain water.
 * Tools: Other examples of technology and tools created by this species were found on Netherop, though when investigated by the Castoffs were unable to be restored to working order.

Karava
This species was discovered by the Covenant on their homeworld of Karava. When encountered, they refused to give in to the Covenant's demands - resulting in the Covenant deploying 300 fighters against the species, wiping it out completely in a total genocide.

Unknown skycrypt
On a world that was controlled by the Forerunners, numerous skycrypts were constructed to keep threats contained. One of these platforms had a security breach leading to the inhabitant escaping.

October 10 aggressors
On October 10, 2552 (ten days prior to the outbreak of the Battle for Earth), a series of twelve unknown starships entered the Sol system broadcasting outdated civilian IFF codes and were engaged by a UNSC Navy task force. Battle Group Rhino engaged the aggressors on October 10, with eleven immediately making unauthorised slipspace transits outside of the Sol system, and the single remaining vessel engaging. was lost with all hands in the battle after suffering numerous hull breaches and boarding actions, before Battle Group Rhino fell back to Phase Line Alpha, over Enceladus.