Aurelians are a bipedal, avian race. They stand taller than most Humans, and have bright yellow feathers and blue eyes, but other color variations may exist. Although resembling birds, the Aurelians do have humanoid arms and legs, and are perfectly capable of using devices like a tricorder. They have wings, though it is unknown if they can be used for flight. (TAS: "Yesteryear")
The Aurelians very much resemble the Skorr, as the animated Aurelian character design was reused for the Skorr as a budget-saving measure. (Star Trek Magazine issue 158, p. 52) In the text commentary to "Yesteryear", Aleek-Om is even described as a member of the Skorr. The Aurelians were not the only species to be reused in TAS; the swooper animations were also used, modified slightly, for the Maravel dragon and mechanical sentinel.
In the script's stage directions, Aleek-Om is described as "a native of the planet Aurelia, and he is a bird-like creature of blue-green hue." The actual episode features a golden color.
The novelization of "Yesteryear" (in Star Trek Log 1) describes the Aurelian Aleek-Om as having an "intricate scroll" – a sign of manhood – tattooed on his beak, "wide, black eyes," as well as a "brilliant gold and blue-green plumage."