The lack of horn wasn't due to laziness or oversight, I just personally prefer my pony cosplay (alliteration!) without horns, ears and tails. I just think they look silly, honestly. So when I designed this look, I went for what Rarity would wear if she were a person - I decided to give her the tiara in order to have something evoke the horn without actually having one. That's the great thing about original designs - designing them to look the way you want is so much fun! And thank you!