Being a demon slayer it's not an easy task, from having great speed to a great strength are the basics recuairements a great demon slayer should poses, a demon slayer riskes his life for others, being capable of not drinking or eating anithing for week, this was one of the strongest cause of death for many , so the leader of the demon slayer corps had no choice then to look for a solution to this.
They would need a companion who is fast ,brave, strong, and able to carry resources, and who's better for the job then a horse.