Unicorns

When lightning strikes a horse, it transforms into a unicorn. Unicorns can be tamed using golden carrots or golden apples, and bred only with enchanted golden apples, making them extremely rare and difficult to obtain. A unicorn would be slightly faster than a horse, but not overpowered. It would also have Slow Falling, allowing it to descend safely from heights, and it could shoot rainbow-like bursts at nearby hostile mobs. A hostile variant could also exist: a unicorn with a Bogged riding it. Instead of rainbow bursts, this version would shoot poison clouds to the player until tamed. Then it would shoot it's poison to the hostile mobs. To keep rarity high, the hostile unicorn would spawn even less frequently than the blue axolotl.