I made a design for that mysterious rhino-like creature in the stained glass window from The Last Problem: derpicdn.net/img/2019/8/31/213… (left window) The writers seem to have left that thing open for interpretation so I decided to come up with my own idea for him.
 
This character’s identity has yet to be revealed and is currently unnamed so we will call him the “Red Beast”. He will be used in an upcoming fanfic I have prepared for “Flurry Heart’s Story”. As you may already expect by looking at the window, he will indeed be defeated by the Student/Young Six but there is a little more to it than that. All will be revealed in time.