Why is the spinal cord shortened in some vertebrates (cauda equina) but not in others?

Hodos, 2009, mentions that the "spinal cord tail" that humans have is not present in most vertebrates. This page mentions cauda equina is not present early on in human embryonic development, but forms later on because vertebral column gets longer. Hodos mentions anglerfish (Lophius piscatorius) also have a "spinal cord tail". What does it look like across other species, like, wolves, gorillas, tigers, crocodiles, dolphins, elephants? Mainly, instead of going species specific, is there any pattern to why some species develop a cauda equina while others do not?

Edit: I can add that according to Uehara, teleosts in general have a spinal cord that extends along the entire vertebral canal, while tetraodontiform fishes have a shortened spinal cord with a cauda equina. This corroborates Hodos claim that most vertebrates have a spinal cord runs the entire length of the vertebral column (teleosts account for 50% of all vertebrates, so… ) Have looked up that dogs have a cauda equina. But, have not been able to understand exactly why some species develop a cauda equina while others do not.

Edit: more context, from Uehara, 2015, "the spinal cord in the majority of the teleosts, the urodelan amphibians, and the reptilian sauropsids and birds extends through the entire vertebral canal, but in the anuran amphibians and mammals is significantly shorter than the vertebral canal."

Edit: more references giving good overview of phylogeny and shortening of spinal cord, but, not really a definitive cause. Nieuwenhuys, 1964 and 1998, and Kuhlenbeck, 1975.

References

Hodos W. (2009) Evolution of the Spinal Cord. In: Binder M.D., Hirokawa N., Windhorst U. (eds) Encyclopedia of Neuroscience. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-29678-2_3171

Uehara, M., Hosaka, Y. Z., Doi, H., & Sakai, H. (2015). The shortened spinal cord in tetraodontiform fishes. Journal of morphology, 276(3), 290–300. https://doi.org/10.1002/jmor.20338

Nieuwenhuys, R. (1964). [Progress in Brain Research] Organization of the Spinal Cord Volume 11 || Comparative Anatomy of the Spinal Cord. , (), 1–57. doi:10.1016/s0079-6123(08)64043-1

Nieuwenhuys, R., ten Donkelaar, H. J., & Nicholson, C. (1998). The Meaning of It All. In The Central Nervous System of Vertebrates (pp. 2135–2195). Springer Berlin Heidelberg. https://doi.org/10.1007/978-3-642-18262-4_24

Kuhlenbeck, H. (1975). The Central Nervous System of Vertebrates, Vol. 4 (Spinal Cord and Deuterencephalon) || VIII. The Spinal Cord. , (), 1–287. doi:10.1159/000395633