The cauda equina is a structure within the lower end of the spinal column, that consists of nerve roots and rootlets from above.
Because the spinal cord stops growing before the bones of the spine, the spinal cord, in adults, ends at about the level of the vertebra L1. To get through the intervertebral foramen at the correct segment, the roots must travel down the length of the spine.
All these roots and rootlets down the vertebral column give the appearance of a horse's tail, which is the meaning of the Latin name cauda equina.