1girl, marvelous_sunday_(umamusume), horse ears, horse tail, elaborate full halloween costume, completely transformed outfit, head-to-toe costume design, distinctive costume silhouette, detailed themed clothing, matching footwear, themed headwear, decorative accessories, intricate costume details, fantasy cosplay outfit, dramatic costume design, original outfit, spooky atmosphere, halloween night, haunted mansion, floating ghosts, bats in the night sky, twisted trees, eerie moonlight, full body, highly detailed, vibrant colors, masterpiece, best quality, absurdres