Borg Species Numbers

The Borg refer to other species by numbers, rather than by names. Their species designations seem to follow the following pattern:

Designations are at least roughly chronological, with the earliest known being three-digit and the latest being five-digit numbers.
Numbers are assigned to any species the Borg encounter, no matter if and how many of them are assimilated or whether they are "distinctive" enough to be assimilated at all.

There is one reference for the number of assimilated species that is accordingly lower than the highest species designation: Seven of Nine says that the Borg have "referenced this condition [of romantic love] in over 6000 assimilated species" in VOY: "The Disease".

Species in Ascending Order

Arturis, Species 116, in VOY: "Hope and Fear"

Species 116 Arturis's species
VOY: "Hope and Fear"

The whole race, except for 10,000-20,000, has been assimilated.

Species 125 Borg Queen's species
VOY: "Dark Frontier"

The Borg Queen mentions to Seven that she came from Species 125.

Species 149
VOY: "Mortal Coil"

This species originally devised the procedure that led to Seven's method to save Neelix with nanoprobes.

Arridor, Ferengi, Species 180, in VOY: "False Profits"

Species 180 Ferengi
VOY: "Infinite Regress"

We don't know why Ferengi are categorized so early in Borg history. Maybe some of them ended up in a wormhole long ago, or Seven was simply confused and meant to say "Species 5180".

Neelix, Talaxian, Species 218, in VOY: "Mortal Coil"

Species 218 Talaxians
VOY: "Mortal Coil"

Seven: "A small freighter, containing a crew of 39 taken in the Dalmine sector. They were easily assimilated. Their dense musculature made them excellent drones."

Species 259 from Galactic Cluster 3
VOY: "The Gift"

Their technology of autonomous regeneration sequencers was assimilated by the Borg.

Species 262
VOY: "The Omega Directive"

Assimilated 229 years ago. According to Seven, Species 262 was "primitive, but their oral history referred to a powerful substance which could burn the sky [the Omega Molecule]."

Species 263
VOY: "The Omega Directive"

Seven: "They, too, were primitive and believed it [the Omega Molecule] was a drop of blood from their creator."

Species 312
VOY: "Natural Law"

They used a similar technology to the energy barrier encountered by Seven and Chakotay.

Culluh, Kazon, Species 329, in VOY: "Alliances"

Species 329 Kazon
VOY: "Mortal Coil", "Relativity"

According to Seven of Nine, "they were unworthy of assimilation."

Species 521 Shivolians
VOY: "Survival Instinct"

Seven and Naomi see a Shivolian in the mess hall, while Voyager is docked at the Markonian outpost.

P'Chan, Species 571, VOY: "Survival Instinct"

Species 571 species of Four of Nine, the younger male drone
VOY: "Survival Instinct"

Four of Nine is against disassembling a dead drone, because this would be against the will of Brothara, a supernatural deity worshipped by Species 571.

Mezoti, Norcadian, Species 689, in VOY: "Ashes to Ashes"

Species 689 Norcadians
VOY: "Ashes to Ashes"

Mezoti identifies herself as being of that species number. We should trust her.

Species 802 Medusans
PRO: "Let Sleeping Borg Lie"

The Borg identify Zero when he connects with the Collective.

Icheb's parents, Brunali, Species 2461, in VOY: "Child's Play"

Species 2461 Brunali
VOY: "Child's Play"

Icheb belongs to this species. The number has not been ultimately confirmed, the information may come from the script.

Species 3259 Vulcans
VOY: "The Raven"

Seven: "Vulcan. Species 3259. Your enlarged neocortex produces superior analytical abilities."

Species 4228 Hazari
VOY: "Think Tank"

Seven: "Technologically advanced, extremely violent. They make excellent tactical drones."

Species 5174
VOY: "Hunters"

Seven mentions that a small ship of that species was found, the passengers killed (probably by the Hirogen).

Species 5618 Humans
VOY: "Dark Frontier"

Borg Queen: "Species 5618, human. Warp-capable, origin grid 325, physiology inefficient, below average cranium capacity, minimum redundant systems, limited regenerative abilities."

Species 5973
VOY: "The Haunting of Deck Twelve"

When Neelix tells of an entity of possibly non-corporeal nature, Rebi suggests that it could have been Species 5973.

Species 6291 "Yridians"
VOY: "Equinox"

Both the Borg and the Federation believed that the species was extinct. Note that these obviously can't be the familiar Yridians from TNG. The ultimate proof can be found in ENT: "Twilight".

Ven, Species 6339, in VOY: "Infinite Regress"

Species 6339
VOY: "Infinite Regress"

Seven: "Species 6339, Humanoid. Warp-capable. Origin: Grid 124, Octant 22 Theta. They first encountered the Borg approximately four years ago. Since that time, 11 billion individuals have been assimilated. Three days ago, the Collective detected one of their last surviving shuttlecraft. A cube was sent to intercept it."The shuttle was purposely infected with a virus that would spread through the Borg vinculum.

Species 6961 "Ktarians"
VOY: "Dark Frontier"

Erin and Magnus Hansen examine a drone on the Raven. "Species 6961...Ktarian...Tritanium infrastructure. He's a tactical drone." Note that this alleged Ktarian looks neither like the TNG nor like the VOY Ktarians.