Scootaloo has a physical disability, she was born with stunted wings and cannot fly. Some people may point out that because one of her parents was an earth pony, it makes her body too heavy for her stunty wings to bear her aloft. I know that many people will point out that Bulk Biceps has stunted wings and can fly, though his flying ability is shown to have varying skill, in one episode he was a wonderbolt recruit and shown to be a capable flier and in another episode he was shown to be a klutzy, slow flier.

Derpy has a physical disability that messes with her depth perception, and early on was depicted as having a potential learning disability, which was later retconned out because some soccer moms complained. She was originally created by an animation error and was kept on because the fans loved her. Her creator has a child with Down's syndrome and came up with the name after the pop culture term derp, which is what someone says when they do something stupid, which was coined by Matt Parker and Trey Stone in their movie Baseketball then further used in South Park. Derpy was always depicted as being upbeat, capable, fun loving and a little bit naive, though she does sometimes make dumb mistakes. I love that the fan base turned her into a hardworking single parent who will do anything for her daughter(s)(Dinky is the most common,though in some fics she also has an older daughter named Sparkler).

According to the creators, Maud is on the spectrum and is the polar opposite from her sisters emotional expression-wise. Maud is shown to be highly intelligent, highly skilled, very loving, very compassionate, highly creative(her imaginary friend boulder is a sign of both high intelligence and high creativity, it may also be a coping mechanism for feelings of social isolation[possibly from growing up on a rock farm in the middle of nowhere, possibly from having difficulty with emotionally connecting with others, possibly from being bullied or teased from other foals her age when she was younger]). I know that her deadpan reactions were originally created as a comedic foil for the exuberant and bubbly personality of Pinkie.

In Rainbow Roadtrip, Kerfuffle very clearly has a prosthetic leg. She may be an amputee or maybe she was born with a stumpy leg.

Friendship is Magic did very well in depicting characters with disabilities as being fully able to live happy, fulfilled and productive lives. All the other characters around them don't treat their disabilities as an impediment, nor do they treat them as if they're made out of glass. I've dealt most of my life with mental health issues and has spina bifida occulta, I know firsthand what it's like to be treated as if I'm made of glass or that my problems are somehow an impediment. I know my limitations and how to work with them.

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Her creator has a child with Down's syndrome

Can you link this group to her ?
https://www.fimfiction.net/group/213632/down-syndrome-group/thread/440570/childrens-book-my-friend-has-down-syndrome

Maud is on the spectrum

I know
Asperger's Bronies United, the Autistic group