Usagi stopped. It caught her off guard. She's been trained to notice her surroundings. Be she didn't even hear the horse's footsteps or the girls voice.
Usagi started running. She knew well enough that the horse's name was Rio but she wanted to know how she got across the field undetected. When she finally caught up with them, she said, "How did you run pass me undetected? I was alert too . . ."
