This page contains information regarding Star Trek: Lower Decks, and thus may contain spoilers.
Standing taller than most Humans, Aurelians were six-limbed with arms, legs, and wings that allowed them to fly. They were covered by feathers that could be various colors including yellow, orange, brown, gray, and white. Their eyes were bright blue. Aurelians typically did not wear clothing. (TAS: "Yesteryear"; LD: "An Embarrassment Of Dooplers")
The Aurelian species was identified by name in the script of "Yesteryear" by way of identifying Aleek-Om. Furthermore, in the stage directions from the script, he was described as "a native of the planet Aurelia, and he is a bird-like creature of blue-green hue." In the actual episode, he is instead 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."
The Aurelians closely resemble the Skorr, as Star Trek: The Animated Series reused the Aurelian character design 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 Aurelian design differs from the Skorr in having a slimmer build, a thinner neck, and longer plumage on the hips. 
According to Brad Winters, Star Trek: Lower Decks uses a design closer to the original Skorr appearance for the Aurelians because it "looked cooler". He has suggested that the Aurelians and Skorr made peace and began to unify after discovering they were related, and thus some of the Aurelians seen on screen may actually be Skorr or Aurelian-Skorr hybrids.