I kind of doubt that this is actually a side ponytail. I'd say it's either a regular ponytail that looks a bit off, or it's twintails with the other tail hidden behind the head somewhat, since those are the two hairstyles you can get in-game. You CAN kind of see some hair behind her right shoulder, so I'm betting on twintails. I'm not changing it myself though since I'm not 100% sure.