Oh! I'll admit, I had completely forgotten about Tirek! You are right, he is a centaur, and I will admit I have only watched the show and never read the comics, I thought at most he was the only one of his species! Thank you for letting me know! I will admit I was thinking more along the lines of the ponies being horrified by the fact that the body had it's head replaced by a human torso. On top of that I was thinking that, if one considers the Equestria Girls movies cannon, then Twilight would have seen Humans and know what they look like, so I felt like it would have been doubly alarming to her to see an amalgamation of a headless pony with a human grafted to them at the waist! Still, thank you for the comment and reminding me about Tirek, I really appreciate it and am sorry that it took me so long to reply~!