Isis
From Memory Alpha, the free Star Trek reference
Isis was the cat that accompanied supervisor Gary Seven from his secret base to Earth on a 20th century mission.
Isis was involved in the 1968 sabotage and detonation of a nuclear bomb in the atmosphere just 104 miles above the surface of central Eurasia. This near-calamity made the rival governments cease, for a time at least, the deployment of nuclear weapons in orbit, and rethink their old strategy of "balance of power".
Though apparently Seven's pet, Isis was more : she communicated with him through a rudimentary telepathy, and understood his spoken words. She accompanied him wherever he went and watched out for him. Isis was also able to take Human form – though which of the two might be her original form remains a mystery. The one thing that never changed was the collar around her neck. (TOS: "Assignment: Earth")
