Here's how getting a disguise for Nyx goes in The Beginning of the Chronicles
Narrator: The next few days, there were no signs of anything out of the ordinary with the foal which Thomas and Twilight had named, "Nyx." However Twilight began to think that they should let Nyx go outside once in a while.
Twilight: Nyx, how about you going outside for a while?
Nyx: Well, I would like to go outside for a while.
Thomas: Wait! I don't think we should have Nyx go outside like that.
Twilight: What makes you say that?
Thomas: Don't you think someone would get suspicious if they saw a black foal with dragon-like eyes and who was an alicorn?
Twilight: [looks at Nyx] Huh, I forgot about that. But we can't have her be in here forever!
Thomas: Well, you could ask Rarity to help make an outfit for Nyx to wear that could cover her wings.
Twilight: Good idea.
Narrator: Twilight took Nyx to visit Rarity.
Rarity: Causal clothing, you say?
Twilight: Yes. Something that can hide her wings.
Rarity: Sure, I can do that! [puts on her glasses]
Narrator: Soon Rarity began threading a vest to hide Nyx's wings. And after a short while, Rarity had created a small purple vest for Nyx and even created a teal headband.
Rarity: There we go.
Twilight: It's looks great, now for the glasses.
Rarity: Glasses? Why does she need glasses?
Twilight: These aren't to correct her vision, it's to make her eyes appear more normal. [she then takes out a pair of midnight purple framed glasses]
Rarity: Aw perfect! This color will go nicely with the outfit!
Nyx: Why do I have to wear a vest and glasses?
Rarity: It's so no one will be jealous of you. Not many ponies here have both wings and a horn, so we're just trying to make you appear like a common unicorn so no one will get jealous.
Twilight: Now, how about I introduce you to our friends?
Nyx: Yes. I'd love to see them!