A/N: Book or movie verse, it's the same to me. Hmm, I don't know about this one. The transition between the end and the final sentence seems kind of awkward to me, but I did my best.
Since the events of that time the seasons have changed countless times, from the bright mien of spring to the harshest of winters, and generations of creatures have risen and fallen—surely enough that her companions of the time, and their children, are long dead. In her lilac wood, however, she cannot observe the pass of time rather than feel it all around her, as even the eternal youth of her forest does not give pause to time—only an illusion of it.
Just as she feels the pass of time, she feels with delight in her heart the presence of her brothers and sisters, scattered throughout the world, often miles apart—but not always. It is the nature of unicorns to be solitary; a crossing of paths between them is rare and accounts for the equal rarity of the birth of a new unicorn. Still, it gives her eternal satisfaction to feel their beauty at the fringes of all things, and to know that she saved them.
Unlike a mortal creature, whose memory fades and blurs with age, her recollection is crisp and clear. She remembers all of their names, their stories and fates. She knows that Schmendrick and Molly Grue were wed, despite being past their time (although one of them did not look it, having once been immortal himself), and had many children and were very happy together. She visited them from time to time, although they never knew it, and had they known they could have boasted the favor of a unicorn. Even Schmendrick's newfound magic could not detect her presence. If anything, both of them experienced sweetened dreams in the springtime, but they kept it to themselves for fear of stoking that longing in the other again.
She knows that Lir had become a good king, took a good wife and lived a good long life rewarded by rendering his father's barren territory fertile and lush. The unicorn can't blame him for it, and wouldn't have even if her heart had been rendered human once more. It doesn't stop her from remembering their love, and that single night where Amalthea discovered human desire and the newness of touch.
Legends say that once blessed with even the elusive sight of a unicorn, one is forever touched by wistful longing for them. But it is the beauty in mortals that has touched her, and left a pressing mark to be intrinsically worn for all the springs that will ever pass.
The truth of the matter is, however, that despite her satisfaction, despite her being wise and beautiful and full of grace once more, Amalthea's human heart beats next to her own, even if only in a very small form. She knows it when solitude becomes too much to bear, and what is more alien to a unicorn, who does not even seek its own kind, to desire company? But it is not the unicorn. It is Amalthea. Amalthea's heart grows weary under the weight of being the only one who knows of what came to pass, and yet simultaneously clings to those memories like a lifeline. She has known love, she has regretted her actions, and she has cried. The other unicorns sense it, and she feels it acutely at times, this alien thing inside her. But then she is herself, and it is tucked away until it surprises her once more.
Such a marriage of mortal and immortal natures have created a strange creature in the unicorn indeed, a mixture never intended by nature. Likewise, she can never forget, even between those moments in which she is more Amalthea than unicorn.
Inside her, Amalthea festers.
Some years later in the lilac wood the unicorn gives birth to a creature with the form, elegance, and immortality of a unicorn, and the eyes and heart of a human.