Tango was a horse 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 they wished Pike to romance. (TOS: "The Cage", "The Menagerie, Part I", "The Menagerie, Part II")