a LOT of design notes for this horse:
-She's tall and graceful but seems very small as she slouches and keeps her wings folded, if you get her to stand up straight with her lankass hair out of the way she actually has a very good model figure which is why photo finish was interested in her. Rarity has also expressed multiple times that fluttershy makes a good model for her dresses.
-Small, weak wings, giving her her signature "flutter". She picks at her feathers to cope with anxiety, causing her feathers to come loose and shed much more often than most pegasi.
-Even after she felt more secure she still plucked and tugged at her feathers (a nervous stim she can't break)
-Her bat wings had pegasi feathers grow over them, but she plucked those out too, and they haven't grown back, leaving her winks pink and bare.
-Now that she no longer has feathers to pull she occasionally bites and pulls at her hair but her family and friends are trying to discourage the habit berore she starts experiencing hair damage or loss
-Rainbow power ALSO permanently affected her mane, giving her a turquoise streak
-Lemon shaped, with little toothpick legs
-Teasingly gets called lemon horse by friends
-eyes always watery because her special "stare" ability causes her to barely blink. Magic keeps her eyes watered, but that means she cries often even if she isn't upset. she has to consciously blink and close her eyes when she sleeps.

Fluttershy has intense social anxiety and feels unsafe around most ponies, and more secure around animals. Her friends eventually helped her feel safer out in public, but she's still jumpy and rather emotionally fragile. Despite this, she tries her best to be brave, especially around her animal friends. She used to be a borderline hermit that nopony in ponyville knew, but now she's become ponyville's impromptu doctor and veterinarian. She also has talents with counselling - you're always welcome to come by dr. fluttershy's cottage and get medicine (or pet a therapy animal for a while!) as long as you're quiet and don't try and engage her in small talk. Her mother is an earth pony and she inherited earth pony magic from her mother's side of the family, giving her her unusual connection to nature as well as smaller wings. Most of her family, in fact, were earth ponies, with only her father being a pegasus. Because of her connection to the natural world, she was drawn to the ground and animals which is why she settled down in the outskirts of ponyville. Her talent isn't actually animal related - that was just a natural gift. Her true destiny is healing.