Lorillian children are born with the ability to breathe only methyloxide via an inhalation apparatus. This means that mother and child must deal with a difficult weaning process when the child reaches age four. Commander Trip Tucker observed a mother and child engaging in this process, and assumed the child was being suffocated. T'Pol corrected him.

Humanoid race, physically distinguished by mitten-like hands and feet — suction cups at the palm and "fingertips" replacing digits and a cranial lobe with an upper set of olfactory nasal openings. Internal distinguishing features include a higher digestive tract, placed where a human heart would be and the Malcorians have more ribs — three on the right and four on the left.
Like humans they come in a variety of skin tones, from dark to fair, and perhaps a little like humans of the late 20th Century, the Malcorians immediately assume both a curious stance and an aggressive stance upon discovering that outsiders have infiltrated their world.
Their technology approximates that of late 20th Century Earth, while their ideology is based on an almost medieval ethnocentrism. They assume their own superiority and believe in their aloneness in the centre of their universe. Despite encouragement from an enlightened scientist, the Malcorian chancellor in 2367 decided to wait for more education before further contact with the Federation, asking not to be visited again.

The Malon was first encountered in 2375 when a Malon freighter had contaminated a vast region of space with Theta radiation. They don't have the technology to recycle their anti-matter waste and have resorted to transport it out of Malon space and dump it at remote parts of the quadrant. They use highly paid anti-matter core workers and labourers to transport their lethal theta-radioactive cargo. Voyager has encountered the Malon at several occasions and they have proven to be quite ruthless.

The colonists and disgruntled Starfleet officers who opposed what amounted to an abandonment of some Federation colonies when the United Federation of Planets-Cardassian border was finally hammered out and "disarmed" are still fighting the "Cardies" a week before stardate 48315, when Chakotay's ship is chased by Gul Evek's Galor-class warship into the Badlands.

A species of highly intelligent noncorporeal beings whose sensory systems differ radically from humanoid sensory systems.  This makes the Medusans particularly well suited for space travel.
While noncorporeal, Medusans still have a physical form, which when viewed by humanoids is said to be so ugly, that it will cause madness in the viewer unless that humanoid has the protection of a visor. In contrast, for those with telepathic capabilities, the Medusan mind is said to be remarkably beautiful.

 Proto-Vulcan humanoids at the Bronze Age level of development, quite peaceful and highly rational, whose evolution closely parallels that of Vulcan. Once cave-dwellers, they now build two-story adobe-like huts, hunt with the bow, weave cloth, use a sundial and have no immediate fear of strangers. A millennia ago they gave up the belief in the supernatural -- including an "overseer" god, astrology and the occult -- but considered it again when a Federation study team was accidentally revealed to them in 2366; to counter the damage, they were reassured of their rational ways and the post was removed. At least some live in extended families; another social custom is a woman's precession of her mate in walking. In appearance they follow the Vulcanoid norm, but their brain has a much lower lysosome level.

An otherwise nondescript race, three of whom are provoked into a fight by a young officer Picard in the roughhouse Bonestell Recreation Facility at the Farspace Starbase Earhart on the old pre-alliance Klingon frontier. Perhaps representative of their cultural level at the time, one stabbed Picard with a serrated-edge metal sword.
Raiders from this large humanoid race broke in to the central museum on Remmil VI using a high-frequency ultrasonic generator to break through the native spun-crystalline webbing used in construction. One is a customer of Garak's on Deep Space Nine, upset when the tailor misplaced his wedding suit.