Recruited by a mad scientist - Elizabeth recruited him to the Plague.
Knew what was in store for you - Xanthos is always having a battle with himself one whether or not he should have joined.
Don't often start things, but you finish them - He stole Mirage.
Your colouring will both go through and not go through the Green Glass Door - Grullo will go through, but slate will not.
Your species will not go through the Green Glass Door - Unicorn will not go through
Your gender will - Stallion will go through, but not a horse (he is unicorn)
The great mare has played you well - A reference to Psyche?