Tosk, meaning "The Hunted", were a reptilian humanoid species native to the Gamma Quadrant. They did not have personal designations, and "Tosk" - once one was aware of the role of the Tosk in their native society - essentially summed up everything about them and their purpose in life.
They were bred and trained to become the quarry in a ritual hunt for another species, known as the Hunters, who would track them, capture them, return them to their homeworld, and ritualistically kill them. Tosk were only sentient because the Hunters made them this way (which implies a degree of genetic engineering on their part). The meaning of life for a Tosk was to outsmart the Hunters for another day. They took great pride in their purpose, as well as looking forward to dying with honor. The only greater dishonor for a Tosk than being captured and returned to their home planet alive was to seek protection from another race. They took an oath of silence regarding the hunt, preventing them from even talking about it with anyone else. The Tosk were greatly valued in their society, and were always treated with respect and admiration by the Hunters.
Tosk were well equipped to outsmart their hunters, presumably to make the hunt as exciting and interesting for the Hunters as possible. Tosk could make themselves invisible to the naked eye (although sensors could still locate them, and the Hunters had a device in their helmets that could scan for them). They also had great agility and only required seventeen minutes of rest per rotation. Tosk kept liquid nutrients stored in plasmic fibers throughout their bodies.
They had the engineering knowledge of how their ships worked, and how to repair them. They could also use computers to get information for tactical purposes.
Tosk were The Hunted, but that did not mean they simply ran for their lives. They were allowed to fight back against the Hunters and even kill them. A Tosk once leapt at a Hunter and knocked them down from a high walkway, possibly killing them. Presuming they could do so successfully, they were allowed to capture the weapons of the Hunters, an energy crossbow, to defend themselves. On the bulkhead of their ships' cockpit above the seat, a dock for the weapon was provided to store (and presumably recharge) it. In one instance, a Tosk knocked out a Hunter and took the weapon, but did not further harm them or attempt to kill them. Immediately after, however, the Tosk was fired upon by more Hunters, this time shooting back and killing them.
When encountering another species, Tosk could be nervous and guarded, but if shown that no harm to them was meant, trust could be built with them and they would become amicable and fairly polite, if tending on the quiet side. A farewell of respect from a Tosk consists of them grasping the recipient's upper arms with their hands and wishing that, when their time comes, they die with honor. (DS9: "Captive Pursuit")
Appendices
Background information
Tosk was described in the script as "a humanoid, but just barely; reptilian skin and bizarre facial features..." [1]
The Tosk make-up configuration was entirely, particularly its scales and coloring, based on the design of an alligator. ("Michael Westmore's Aliens: Season One", DS9 Season 1 DVD special features; Star Trek: Aliens & Artifacts, pp. 148 & 150) Remembered Make-Up Supervisor Michael Westmore, "The script described Tosk as a 'reptilian-based alien.' I was thinking we'd already done snakes and lizards when I happened to glance at a National Geographic that had an alligator on the cover, and I thought it would be interesting to do the character as an alligator type." (Cinefantastique, Vol. 24, No. 3/4, p. 106) The back of the Tosk head - with its dark greenish color and large, thick scales and ridges - matched the back of an alligator, whereas the front of the Tosk head, featuring a lighter yellow-green and smaller scales, was like the belly of an alligator. ("Michael Westmore's Aliens: Season One", DS9 Season 1 DVD special features; Star Trek: Aliens & Artifacts, pp. 148 & 150) Westmore commented, "We just took a big chunk right out of the middle of an alligator and put it on his head." Although the look of the Tosk was intended to be easily identifiable as reptilian, the degree of similarity to an alligator was intended to be subliminal for the audience. ("Michael Westmore's Aliens: Season One", DS9 Season 1 DVD special features)
Once the inspiration from an alligator was applied, the actor playing the Tosk was then fitted with "slit" contact lenses, to make the alien character look even more reptilian, as well as tiny fanged teeth and gloves to represent the Tosk's hands. Due to the Tosk costume in "Captive Pursuit" having a low neckline, the make-up design included a layer of scales for the performer's upper chest. (Star Trek: Aliens & Artifacts, p. 150)
The Hunter stated that the Tosk would be taken back alive, and put on display, where children could "make fun of him, and throw him scraps of food." While this suggests that Tosk did need to eat, it is also possible that the scraps were meant to humiliate the Tosk, akin to throwing rotten tomatoes at bad stage performers, and that a captured Tosk would not actually eat the scraps.
It was possible that the Tosk and the Jem'Hadar are somehow related, as the two races share traits, as well as their cloaking ability. The script for "The Jem'Hadar" stated that the shrouding ability of the two was to be identical and implied that the Tosks were bred by the Founders of the Dominion, as gifts for the Hunters. Robert Hewitt Wolfe speculated that the Tosks were supplied to the Hunters by the Vorta. (Star Trek: Deep Space Nine Companion, p. 154)
According to the reference book Star Trek: Star Charts (p. 74), the system where this species came from wasn't identified. Their homeworld orbited a Class K star. It had a magnitude of +9.
Jake Sisko, Odo, and Opaka encounter a Tosk in Rising Son.
External links
Tosk at Memory Beta, the wiki for licensed Star Trek works