Mary Lou was one of two horses owned by Starfleet Captain Christopher Pike at his home in Mojave, the other horse being Tango. Pike often rode out to the parkland surrounding Mojave with Mary Lou and a picnic lunch.

In 2254, while attempting to mentally manipulate Pike into accepting an illusory reality, the Talosians inserted a telepathic scenario into his mind, wherein he found himself back home in Mojave with Mary Lou, Tango, and a version of Vina, a Human woman whom the Talosians wished Pike to romance. (TOS: "The Cage", "The Menagerie, Part I", "The Menagerie, Part II")

Their name originated from a scripted source. In the various teleplays for "The Cage", Mary Lou was to have been named on screen by Pike, when he said hello to her, while patting her. He also would have given her sugar that he gave to Tango, too. In the final version of the episode, though, he doesn't speak to her at all and instead gives the sugar only to Tango.
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