The Lactrans were large non-humanoid creatures, moving in a slug-like manner. On the front of their face was a trunk or tentacle type structure which ended in three finger-like ends, which they used as their main mode of object manipulation. Their two yellow eyes were slightly raised from their head.
They were a highly advanced species, telepathic and furthermore thinking so fast and in such a complex manner that Vulcans could not follow and Humans might go mad from interacting with them. A six-year-old Lactran already had an IQ in the thousands.
Their telepathic powers could be used to invade other minds and probe them for information. One Lactran might not be strong enough to break into a Human mind, but they were able to combine powers. (TAS: "The Eye of the Beholder")
The Lactrans kept a large zoo of lifeforms from other worlds on their planet.
After telepathic communications, Spock stated that they were once at a similar level to Humans, "tens of thousands of centuries ago... their centuries." Lactrans, after absorbing information about the Federation, concluded that it was simplistic, but in the process of evolving into a higher order - even Vulcans.
A difficult but ultimately successful first contact between the Federation and the Lactrans was made in 2269. The USS Ariel was the first to explore the planet, but its crew were all killed or captured. However, when the USS Enterprise arrived to investigate their disappearance six weeks later, they were also captured, but ultimately managed to communicate with the Lactrans and get themselves released.
Technologically, the Lactrans built at least once city covering an estimated five square kilometers, which was considered an admirable feat. They possessed advanced force fields, for which a Constitution-class ship was no match and which were completely invisible, even when touched. They were also believed to be able to travel to other star systems. They had also terraformed parts of their planet into custom environments for their zoo. (TAS: "The Eye of the Beholder")
Although it was felt that Starfleet had manage to learn practically nothing about the species, the Lactrans made it clear that they were welcome to revisit the planet, in twenty or thirty centuries. However, this was again in Lactran centuries, and it was not immediately clear how long that was. (TAS: "The Eye of the Beholder")