The Phylosians were a species of sentient plants who were native to a planet they called Phylos.

The Phylosians developed into a technologically advanced civilization. They had a naturally long lifespan, which was further extended by their greatly advanced medical science. At some point in their history, the Phylosians set about the task of enforcing peace in the galaxy. They built a fleet of large Phylosian starships, intending to use them for this task, but other events intervened.

Stavos Keniclius, a Human scientist, visited Phylos in search of what he called a perfect specimen. However, Keniclius was a carrier of a non-native, staphylococcus strain of bacteria that nearly destroyed all life on Phylos. He worked to save the Phylosians from his disease. Thanks to his work during the Eugenics Wars, he was able to create many clones of himself, in an attempt to undo the damage he caused. The Phylosians eventually referred to Keniclius as "the Master." When it became known that Keniclius had the same goal of imposing peace throughout the galaxy with a master race of clones, the Phylosians agreed to help Keniclius in acquiring the perfect specimen to copy.

When the USS Enterprise visited Phylos in 2269, Stavos Keniclius 5, the fifth clone of the original Stavos Keniclius, chose Spock as the perfect specimen. He cloned Spock, creating Spock Two. However, the galactic chaos that Keniclius had hoped to combat no longer existed, thanks to the work of the Federation. Keniclius decided to give up the plan to create the army of clones, and instead planned to work on the problem of saving the Phylosian species. Spock Two also remained behind to help Keniclius with this task. (TAS: "The Infinite Vulcan")

A Phylosian sat on the Elysian Council. (TAS: "The Time Trap")

In the script for "The Infinite Vulcan", Phylosians are initially described thus; "They are Human size, but are not humanoid. Their heads are partially covered with a furry bristle... eye stalks but no mouths or ears are visible. The bodies are composed of a group of slender rope-like extensions, some of which hang down at the sides, and others of which bunch near the bottom before spreading out again into a loose collection of bulbous protuberances which are apparently the motive limbs. The body color is an ochre yellow, almost sickly. Despite their alien appearance, they seem to be passive and harmless; the voices we hear [from them] are quiet and reasoning." The script referred to Phylosian corpses as "giant twenty-five foot bodies covered with some kind of furry bristle." The script went on to say about the dead Phylosians, "There is no head as such, but at the top end is a mass that looks like an artichoke. Each body is composed of a series of furry rope-like extensions, bunching near the bottom and spreading out below that into a group of bulbous protuberances. The similarity of Agmar and his aides is obvious... but so are the differences."
The novelization of "The Infinite Vulcan" (as published in Star Trek Log 2) notes that the external appearance of a living Phylosian had no similarities to that of Humans or Vulcans, except in size.