Kira is the *top* Equestrian

"I have a simple test that can help you make the decision, ladies. Interested?"
They both nod.

You really hope this works out like you planned...

"Audrey, lightly grab Kira's hair and pull her head back. Just trust me."

She slowly does what you suggest and Kira doesn't seem to like it.

"Alright, now try it the other way around."

A more confident Kira grabs Audrey's hair and pulls it back.
She smiles at her blond friend.
A sultry smile is answered.

"There's your answer. One of you is what is called a 'Top.' You like to be in charge.
That's you, Kira."
"Well...that does seem to track," the Brunette says.
"So what does that make me," Audrey asks.
"You're a Bottom. You like to take commands- from people you trust, that is. You like to help out and to serve."
"That is totally you," Kira says.
"Guilty. So...I'm the Horse then?"

You and Kira nod.

You leave the ladies to get changed, but, of course, film the whole thing for your private collection...err, research.
Yeah, research. That's it.

After several minutes, you knock and are granted entry to see the 2 Actresses in their new roles.