Unicorns have been part of many cultures, usually being a symbol of purity and grace, which could only be captured by a virgin—another reference to purity. Its horn was believed to have magical powers, like to render poisoned water potable, and to heal sickness.
Rainbows have formed an important part of human culture since the beginning of civilization. They have been associated with many meanings, such as: signs from the gods, good fortune, peace, etc.
As you can see, a Lady Raincorn costume seems like a good choice, regardless whether you’re a fan of Adventure Time or not!