I think Cosmos actually has an ability that Discord may not have, and that’s possession. I’m not positive he didn’t, but did Discord ever use this? I feel like he probably would have if he could. She’s not just taking on Twilight’s form, if I remember correctly, she took over her body.

Now what if she took over EVERYPONY’S body, or at least as many of them as she needed, and used them like her own little Changeling army, but with the REAL bodies. (Zecora’s little mud mask can’t disprove that. It IS them.)