Man do I love Discord.
He's like a living play-doh fun factory that I can work around into whatever I want.

Pony forms:
Adolescent Discord: When Discord was learning the ways of Equestrian society and pony life, Starswirl the Bearded demanded he assume the form of a pony while in public. He always made a point to state he hated the way Discord looked. So Discord tried to blend in as best as he could when he was still learning to control his magic.
Show-aged Discord: Shortly following his reform he occasionally wandered about Ponyville in this form, that way every pony knew it was him yet he wasn't a giant string bean creature.
My Headcanon Era Discord: As a professor at the Canadursan National University Discord finds it easier and less strenuous to give lectures in an auditorium when he can calmly stand behind the podium and speak into the microphone. His students appreciate it sometimes too because his lengthy body won't block the screen and projector. He chose to settle with a Earth Pony for and held a contest for extra credit, asking his student to think of a cutie mark for him to use. He had nearly 500 entries and the one he chose was actually the most popular submission. He also will kiddingly introduce himself to new classes as Dr. Enthalpy or Dr. E. No one really calls him discord accept for his close friends and the president of the university. everyone else will refer to him as either Dr. E or Dr. D.