Unicorns and Unicrons

What is the difference between unicorns and unicrons?

While waiting in line at Half Price Books, there was book titled Unicorns on one of the main display islands. Since I often see it misspelled (and often mistype it myself), I asked Olivia if she knows about unicrons, and pointed out the book to her. She corrected me that it was indeed unicorns, and not unicrons.

So, I asked the difference between a unicorn and a unicron.
(Because come on, let's face it, a unicron sounds pretty cool too. Like some sort of supercomputer unicorn or something. "All Hail the Mighty Unicron!")

So, it was explained to me that a unicorn is a horse with a horn on it's head, and a unicron is a pony with a horn on it's head. A pony is like a horse, but smaller.

I can't very well argue that she's wrong, so that's what a unicron now is.