Polymorph only transforms into Beasts, and I doubt horse girls count. He'd need True Polymorph, a 9th level spell. In 3.5, you could do it with just Polymorph, but they'd retain the mental ability scores (Int 2, Wis 12 or 13, Cha 6). So, not that cute, and not smart enough to talk. Polymorph Any Object is 8th level instead of 9th level so it's a bit more accessible, and it would give her an Intelligence of her new form. Though I'm not sure how that works since NPCs aren't monsters in 3.5. I guess just roll up their stats?
Only thing I can think of is Nystul’s Magic Aura to make them count as Humanoid, followed by Reincarnate. But they have to die to make that work.