A mature vrell was at least ten feet tall, had six-inch-long claws and three horns on their head. It was strong enough to break thick chains. Its level of intelligence was similar to that of a horse or dog, but it was able to create telepathic links between people who were nearby. Its favorite treat was Solithian asparagus.
In 2266, a mature vrell was brought to Urey University for study by xeno-zoologist Barbara McCoy. Visiting Captain James T. Kirk experienced the vrell telepathic bridge firsthand when he was near McCoy and the vrell. She knew he wanted to ask her out, and he knew she already had a date.
While they were at the theatre, Spencer, a university guard, taunted and whipped the vrell, prompting it to break out and crush him. It escaped from the grounds, then was pursued and fired upon by armed hovercraft. McCoy appealed to Colonel Mitchum to let her handle the animal. She and Kirk climbed up Galileo Tower. She reached out to calm the animal but over-extended and fell over the side of the stairs. The vrell leaped and caught her, carried her back up to Kirk with a final, mortal effort, then fell off the tower. In its last few seconds of life, it relayed a telepathic message from Montgomery Scott that the USS Enterprise was in jeopardy. Kirk and Leonard McCoy were able to defuse a bomb set by criminal Red Garvin and save the ship. (TOS comic: "Furlough to Fury")