The mystery of this gif, a thread
This bag can't be used as evidence for this gif because literally everyone in sixteen has one (attached the pictures below)
The hair cannot be confirmed black, right at the end the hair is a similar shade to Jeong's brown but at the end of the day its all about lighting.
In the chance that it IS brown and meaning it can be Momo, refer to above about the bag. As for the shoes, Sana also has the same pair of shoes.
"Sana and Jeongyeon were not close". Attached below is what Jeongyeon wrote about Sana during Sixteen era. Sana can also be seen in 6mix photos.
A moment from sixteen
And another
And in case none of that confirms to you, here is a tweet from a k-once
https://twitter.com/graycatjji/status/619058480012398592
https://twitter.com/graycatjji/status/619058480012398592
A tweet from a j-once featuring an insta post from a k-once https://twitter.com/apr_peach9/status/624952376777011200
And Twicepics https://twitter.com/TwicePIC/status/641657600027717632
The gif of it, eliminating who you're thinking of
Its Sana