(covers information from several alternate timelines)
This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Kelpiens were an intelligent humanoid species with a tall, lanky build, a brownish orange skin, and no facial hair. Tendril-like threat ganglia splayed out from the back of a Kelpien's head when they sensed danger. The Kelpien population was low. (DIS: "The Wolf Inside")
The species map on their homeworld did not have food chains, but instead was a binary of predator or prey. Kelpiens were the latter; they were hunted, bred, and farmed, like Humans' livestock of old. (DIS: "The Vulcan Hello")
According to Saru, Kelpiens lived in a constant state of fear from the moment of birth. Due to being a prey species, Kelpiens tended to prudently avoid risk or dangerous situations, giving them a reputation as being somewhat cowardly. When faced with a novel situation, their default assumption was typically to consider that it might be hostile – i.e. if it was unknown what had destroyed a relay satellite, and it could plausibly have just been destroyed by an asteroid impact, a Kelpien would first assume that it was hostile action by a nearby threat, until proven otherwise. This did not mean, however, that Kelpiens would enter a fit of panic when faced with danger; rather, when faced with a fight-or-flight situation, they would calmly insist that flight was the wiser course of action.
This heightened sensitivity was trained into Kelpiens through their culture, but the species was also biologically determined with heightened senses, a hyper-vigilance to detect approaching danger in order to flee it. This had been honed to the point of essentially a sixth sense warning that a situation was dangerous – as Saru put it, the ability "to sense the coming of death." Besides having more developed senses, Kelpiens were also faster and stronger than Humans. (DIS: "Choose Your Pain", "The Vulcan Hello", "Si Vis Pacem, Para Bellum")
In the mirror universe, the species was both slave and livestock to the Terran Empire. Their entrails were dined on, and their ganglia were prized particularly. (DIS: "The Wolf Inside", "Vaulting Ambition", "The War Without, The War Within")
List of Kelpiens Edit
Background information Edit
The portrayal of the Kelpiens was originated by a collaboration between the writing staff of Star Trek: Discovery and Saru actor Doug Jones.  The opportunity to develop a new species was extremely exciting for Jones. He was glad he didn't have to play an already established species, because doing so would have had some pressure that, if he didn't get the portrayal right, Star Trek's fan base would have hated him. By contrast, he said, "This is like, 'Well, no one is going to know if I'm getting it right or wrong because, it's starting right here!'"  On the other hand, he did feel that the responsibility of establishing the Kelpiens was somewhat "daunting," because he wanted to do so in a way that was "right" and could be "emulated in later incarnations down the road through someone else." 
Doug Jones likened the Kelpiens to gazelles, saying that was "the best analogy in nature." He remarked, "We have our moments where we're chewing on something green and at peace." However, Jones also pointed out that Saru's people were meanwhile ever aware (by necessity) of danger, the actor comparing the species that hunts them to lions. 
The Kelpien makeup primarily consisted of a silicone prosthetic cowl covering. A facial appliance and separate lower jaw and lower lip pieces were then added and blended together to complete the head. Prosthetic gloves were used to modify the hands. (After Trek: "Episode 2" – "Becoming Saru") The Kelpien makeup scheme was designed by Neville Page. However, the makeup scheme included only one digitally enhanced component (the threat ganglia) and no mechanically enhanced parts. "Nothing else has to be mechanically enhanced," Doug Jones clarified. "There's no servos, there's no joysticks, there's nothing that needs to be puppeteered or CG enhanced, except for the threat ganglia [....] This is a [real] gift to the budget, to not have to use that much CG." The absence of CGI enhancement accentuated the importance of the actor's performance in how the alien was portrayed.  According to Star Trek author David Mack, the color of Kelpien eyes was added in post-production (at least for Saru's first couple of appearances) and wasn't visible in the initial set photography. 
The depiction of the species was influenced by Doug Jones devising Saru's physicality, such as the actor deciding how the character would stand, walk, and move his hands. For example, Jones decided Saru would walk "very much like a supermodel" and have hand gestures which he described as "very fluid" and "very much like an animal."  Jones found developing the look and physicality of the species was "really fun."  This physicality was enhanced by the use of special boots which lacked a heel, putting Jones up on his toes and adding an additional five inches to his height. This had the effect of changing Jones' center of balance, leading him to develop the "supermodel" walk. (After Trek: "Episode 2" – "Becoming Saru") In fact, according to Jones, they were meant to suggest that a hoof was inside. (After Trek: "Episode 3")
According to Doug Jones, no other members of the species apart from Saru had served as a Starfleet officer prior to him doing so. (; After Trek: "Episode 2" – "Becoming Saru") Jones is unsure but curious about what happened to this species after the events of Star Trek: Discovery. "I don't know if it can live on after this series because this is 10 years before The Original Series, so we'll see. It will be interesting to see what happens [...] because, if this species never showed up anywhere after this in the timeline, the question remains, 'What happened to the Kelpiens?'" 
According to the novel Desperate Hours by David Mack, Kelpiens are from a world known as "Kelpia". The novel suggests they spend much of their time living underground, having been driven there "by the coming of the t'rrask" and that Saru was rescued from the world by Starfleet officers.