Why did L'Engle choose Gaudior to be a unicorn?
Gaudior, as a unicorn, symbolizes innocence and purity. Thus he constrasts with the darkness that the children encounter. In the fight of good against evil, the purity of the unicorn will provide the children, presented as good though with the normal human flaws, to overcome those flaws in order to complete the battle.
Gaudior also aids Meg and Charles Wallace in kything, which is a kind of mental telepathy. Through him they achieve the level of communication that is necessary in this epic battle to set all things right, at least as much as they can be in a fallen world.