Charlotte was new to the casting of spells.
The cadence of her reading wasn't quite right. Her tonality was off a bit. She hadn't developed the full magicks of a veteran spellcaster.
But, this was still real magic, and she wasn't to know all those details yet as she cast the spell on Amy.
Amy was still asleep when the changes started, and slept right through the transformation. When she awoke, she felt odd, even before opening her eyes. Trying to stretch, everything felt wrong, and she slowly, with some dread, started opening her eyes and looking around. She looked down at her hands, and couldn't see where she wanted to look, and when she tried to move her arm and spread her fingers wide, the sensations were all wrong. She saw, with altered and what felt like weakened eyesight what was obviously a horse's pelt, and her heavy head and odd-feeling orientation of her eyes...well, she had a quick idea what was happening, even as her brain didn't let her accept it.
She went to the light switch on the wall, determined to stay calm, and used her nose to turn on the lights. She saw an odd kind of horse in the full-length mirror on her wall. A scrawny neck, patches of human skin in the front half of the body, a mouth that didn't look right with the head, and...she felt she must still be half asleep...four prominent stallion phalluses at attention. She'd been around horses before, and had to say they didn't look all that impressive for size, and looked, well, halfway human, too. That had distracted her from the fact she also had eight legs, but she quickly got a grip, took inventory of what she could still see, found out she was also still a girl...well, a mare...where that might have counted, at least for what she could see of it, and decided to try to get help. This had to be some kind of bad dream.