Lucy's Horse is a pet that used to own by a Violette1st character (which is, obviously, Lucy). She never even been seen in one video, but was mentioned by Lucy in a QnA.
According to a QnA, Lucy said she used to have a horse. Although she couldn't remember her name, she didn't like the horse. She did sold the horse to a circus in Dallas, TX and she was taught tricks. Once she mastered the tricks, she would spend the rest of her life in the circus until she died from a unknown disease.