Sleep isn't real.
@cheezedoodle
Prehaps. It’s really weird since they either forgot her horn by accident, just had her as a one shot background character and never thought about it again until she started to become a more developed character (although I’m not sure if which came first, Return of the Crystal Empire or the comics that shows her as a unicorn), or Hasbro just asked them to make her a unicorn. Regardless, while I’m usually okay with these sort of things since there’s usually a lot of variations, here it’s literally just one small detail that is missing from an otherwise exact same design.
Looking at
screencaps from the show, I think it might be the first option. She has appeared multiple times, though she os sometimes missing her horn. Not only in Ponyville either, her most recent show appearance was in “Spice Up Your Life”.
In Canterlot. This happens all the time with background characters, so I wouldn’t be surprised if this was one of the cases where it might be an error.