Star Trek: TOS, S1E5 - The Enemy Within
The episode starts out with Sulu holding some king of horned-lion-dog with antennae and a scaled spine. He's cute and rocking the costume but no preamble is given. We're not told what the creature is, if it's indeed a dog, or why it was shoehorned in the episode. Alas, the nameless horned-lion-dog looks on as Sulu and Kirk complain about how the planet they're on will become unbearably cold. Clearly the animal isn't phased and we end up seeing him a lot more in the episode yet he never gets identified. He is simply horned-lion-dog. An injured crewman beams up with some kind of yellow ore residue on him that effects the transporters. When Kirk beams up and exits stage left with Scotty, a second Kirk follows who is apparently evil by his over exaggerated facial expressions. Evil Kirk literally creeps around creature-like, becomes drunk on Saurian Brandy, and tries to force himself on yeoman Rand. She claws him and luckily escapes to report what happened.