Capellan power-cat
A Capellan power-cat was a fierce, feline creature native to Capella IV. 

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." ()

Background information 
In the audio commentary for "How Sharper Than a Serpent's Tooth", David Wise explained that "Russell and I were writing the script, we thought there has to be a sort of wild, ferocious, very dangerous creature so that when they smash all the cages, it'll threaten Kukulkan, our guys can handle it and Kukulkan can realize maybe we're more grown-up than he thought. So I had this cat, this orange tiger-striped cat named Nick, and he was a sweetheart, but he was very grumpy on the exterior, but then when you got to know him, he was a big bag of mush. And, basically, he sort of walked across the floor, and Russell and I both looked at him and went, "There's our animal." And we came up with this idea of a combination between, in essence, a lion and an electric eel, which sounds reasonably science fictional, called a Capellan power-cat."

de:Capellinische Energiekatze
Felines