Small dog-like animals native to the planet Alfa 177, one of which is split into a docile and a savage twin by the Enterprise transporter in The Enemy Within.
The Alfa 177 canine is a small, furry animal with a horn and a spiny back, found on the planet Alfa 177 in the original series episode The Enemy Within. The creature was portrayed by a dog in costume.
In 2266 a specimen is beamed aboard the Enterprise shortly after a technician covered in magnetic ore dust has upset the transporter. Two animals materialise, one gentle and one vicious, which gives Scott and Spock their first proof that the machine is dividing whatever passes through it, as it has already divided Captain Kirk.
When Scott believes the fault is repaired, the two animals are sent through together as a test. They return as a single creature, but it is dead, apparently from the shock of being rejoined, which makes the attempt to restore Kirk a dangerous gamble.