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Two Nimbosian horses

"Yes, Captain!"
"Be one with the horse!

Nimbosian horses were blue-colored riding animals by the Galactic Army of Light on Nimbus III during the 2280s. They were blue, with a single horn on its forehead, and could be subdued by a Vulcan nerve pinch. No less than ten of these creatures were found on this planet.

Sybok's army used horses to attack Paradise City, as did a Starfleet raiding shortly thereafter. (Star Trek V: The Final Frontier)

Background information

Close-up of horse being colored

According to the reference book Captain's Log: William Shatner's Personal Account of the Making of Star Trek V: The Final Frontier it was noted throughout that these creatures were identified by the production staff as "Nimbosian horses".

The final design concept art, dated 22 February 1988, described the beast simply as a "Nimbus III Horse." These "blue unicorns" were the brainchild of writer, director, horseman, and horse breeder, William Shatner, and according to Nilo Rodis-Jamero, "from the very beginning, he suggested we make Sybik's mount an alien unicorn. So that's what I did, although I did add an extra horn, like a rhino. I incorporated the horns into the aminal's bridal–that was it's costume, for all intents and purposes. And later on, we made it blue, to give it a more other-worldly quality." (Star Trek: Costumes, p. 80-81)

The novelization of Star Trek V: The Final Frontier refers to the alien horses on Nimbus III as "tsemus" ("tsemu" in the singular), though they were plainly called "horses" in the film's script. [1]

In an ultimately discarded scene from the final draft script of the Star Trek: The Next Generation episode "Family", a brief mention was made of an unnamed member of the Crusher family who had been a horse thief on Nimbus III. (Star Trek: The Next Generation Companion (3rd ed., p. 143))