Someone in class wrote up a wonderfully detailed and informative blog post about the story of the Lion and the Unicorn as told in Through the Looking-Glass. There's all kinds of information and illustrations, too, showing how Tenniel modeled his own drawing for the scene on Disraeli (the unicorn) and Gladstone (the lion) - here is a link to her blog post: A Little Background on the Lion and the Unicorn.
I thought I would contribute something from way out in left field - a very obscure (and I mean very very very obscure) Aesop's fable about the lion and the unicorn. As far as I know it is recorded in only one written source, the fables collected by John Sheppey, Bishop of Rochester, in the 14th century (he could have gotten it from a more complete version of the 13th-century fabulist Odo of Cheriton than what we have now - but, if so, that version by Odo is lost and all we have is what is recorded by John). Anyway, here is the story; the Latin is down below and I've included an English translation here. It's part of a project I did two years ago, Mille Fabulae et Una: 1001 Aesop's Fables in Latin. (The Latin text has been slightly abbreviated, since I limited each fable in the book to 125 words or fewer.)
The Lion and the Unicorn. The lion pretended to be sick and, limping, he went up to the unicorn, his chief enemy, and greeted him. Then he said, "Whatever has taken place between us up until now, let it be forgotten, because I will not be able to do anyone any harm ever again, as you can see, since I am old and suffering from all kinds of infirmities. But before I die I greatly desire to speak once more with my wife, who is away in the desert, and I would ask you to loan me your horn, if you would be so kind, so that I could use it as a crutch on my journey, because your horn is quite long and strong. I will send it back to you as soon as I reach my wife, and on that I give you my word." The unicorn believed everything the lion said and felt sorry for his suffering, feigned though it was, so he gave the lion his horn, leaving himself defenseless. The lion then advanced on the unicorn and sprang; seriously wounding the unicorn with his own horn, he thus defeated him.
Leo et Unicornis. Leo, fingens se infirmum, obviavit, claudicans, unicorni, adversario suo capitali et salutato eo dixit, “Qualitercumque actum fuerit inter nos hactenus, remittatur hinc inde, quod ego ulterius nulli nocere potero, prout vides, senio et variis incommodis debilitatus. Sed multum affectarem semel loqui cum coniuge mea, quae est in deserto, ante meam mortem et peterem a te ut accommodare mihi velis cornu tuum pro podio habendo in itinere, quia satis longum et forte est. Tibi remittam illud quam cito ad coniugem pervenero, et ad hoc tibi do fidem meam.” Unicornis vero, dictis eius omnibus credens et ipsius confictae miseriae compatiens, commodavit cornu suum et sic remansit inermis. Leo vero, modicum progrediens, fecit insultum in unicornem et, proprio cornu graviter vulnerans, devicit eum.
Image Source: Wikipedia. This is the lion and the unicorn as shown on the royal coat of arms of the United Kingdom.