Much to the chagrin of zombies everywhere, this is not —and I quote–“BR-R-R-RAIN-NSss”, but rather a “brain cactus”. This is a morphological variant of the species Mammillaria elongata DC., or golden star cactus. The distinct morphology of this and other brain cacti, known as cristate or crested growth, is caused by an apical meristem gone awry.
The shoot apical meristem is the site of growth that elongates the plant. It’s located at the growing tip (apex). In the case of this cristate variant, the apical meristem no longer performs cell division from a single point. Instead, the formation of new tissue occurs as cells divide laterally from an elongated meristem. For other examples of cristation, see this PDF of cristate saguaro cacti (you can also read some speculation on the whys of cristation in saguaros: Crested Saguaros – What is the Rhyme or Reason?).
Botany resource link: Linked above, Cal’s Plant of the Week (by the late Cal Lemke of the University of Oklahoma). Cal was the botany greenhouse grower, so the plants he featured tended to be tropicals.