While Fluttershy walked near her cottage she found new strange flower. She came to Zecora to ask about it but zebra couldn't recognize it, even Twilight's books didn't help.
So Fluttershy just started examining flower. Until it hypnotized her and she ate it. This was Discord's last (probably) surprise. Lesson for you: don't eat everything that wants to be eaten (unless it hypnotizes you).
Why Flutters? Because she loves animal WAY too much (I also wanted to do something with her).
List of parts:
Front legs - Bear (I hope it looks so)
Back legs - Goat (like Discord's)
Ears and wings - Bat
Tail - Squirrel (There's must be something little and cute)
Eyes - Cat
She won't always be that angry (and evil), it's just post-mutation period, she can't control herself yet and releasing all the bad things that accumulated in her for this time.
After that, she'll think she's monster and go hide somewhere in deep hole so nopony will find her, but animals' ll calm her and everything will be fine as usual (if it can be now).
Also, now she's fearless to Everfree and all its creatures (probably).