In appearance, the power-cat had thick auburn colored fur, stands about a meter tall, and had a row of black spines on along its backbone. The power-cat had the ability, similar to that of an electric eel from Earth, to create and discharge electricity when angered. If a potential captor attempted to "put more than one hand on [one]", they would be subjected to a charge of 2000 volts.
The power-cat was extremely untamable, but above all, hated captivity. In fact, according to Dr. McCoy, "no one has ever been able to keep one alive." Kukulkan, however, had a power cat in his zoo that he had captured as an infant and thus, was able to easily control it, and later placed it into a caged-simulator that mentally placed the power-cat into its own natural environment to live.
In 2270, when the crew of the Enterprise encountered and were subsequently abducted by Kukulkan, they discovered his power-cat. During an attempt to distract their captor to give Spock, aboard the Enterprise to formulate a defense against Kukulkan, they pulled the cable on the power-cat's cage.
The power-cat, revived from its captivity, began to threaten the rest of the animals in the room. Kukulkan was unable to control the now-free beast, because the Enterprise had since disabled his ship's central power source.
In a quick fix, Kirk had McCoy prepare him a hypo, set to full tranquilizer setting, and injected the power-cat. Kirk then successfully injected the power-cat, but not before getting a small power jolt from the cat, which knocked Kirk to the ground.
Although not knocked out itself by the tranquilizer, the power-cat was left in a condition where it was laying down cleaning itself while in a demeanor, best described by Ensign Walking Bear as "just like a big kitten." (TAS: "How Sharper Than a Serpent's Tooth")