I love this and I'm planning on playing one in an upcoming Hell's Rebels game where I can use the change Shape ability to blend in and then horse-out when combat comes. In particular though I'm considering playing her as a Life Oracle. So I do have a mechanics question, what size am I when starting off in a regular class? I didn't spot anything about size starting out as a non-paragon class.
That is because Spot is a dog.
Perception check DC 10
Re'em racial trait (second one)
Small: Re’em are Small creatures and gain a +1 size bonus to AC, a +1 size bonus on attack rolls, a -1 penalty to their CMB and CMD, and a +4 size bonus on Stealth checks.
So unless you take either Re'em Hero universal archetype for oracle, (or level up like a real unicorn, silly two-legs, with the Silvermane Exemplar) you will stay small (Like Uni).
Hope that helps.