Tango

Tango was a horse presumably owned by Starfleet Captain Christopher Pike at his home in Mojave. Tango was fond of sugar cubes, and Pike often kept a supply in his jacket pocket. Pike often rode out to the park land surrounding Mojave with Tango and a picnic lunch.

In 2254, while attempting to mentally manipulate Pike into accepting an illusionary reality, the Talosians inserted a telepathic scenario into his mind, wherein he found himself back home in Mojave with Tango, as well as a version of Vina, a Human woman whom they wished Pike to develop romantic feelings toward. (TOS: "The Cage", "The Menagerie, Part I", "The Menagerie, Part II")

Evidently, Vina had been riding Tango that day. Vina tells Pike that the coffee thermos is hooked to her saddle; Pike then goes to Tango to retrieve the thermos--not to the other horse that is visible in the scene.

In the second revised final draft script of "The Cage", the second horse present in the scene is given a name: Mary Lou.