It actually IS called a "thagomizer"

This week The Far Side reposted the classic 1982 cartoon embedded above.  Many people don't realize that Gary Larson's neologism has now been incorporated into the scientific lexicon:

Thagomizer has since been adopted as an informal anatomical term, and is used by the Smithsonian Institution, the Dinosaur National Monument, the book The Complete Dinosaur and the BBC documentary series Planet Dinosaur. The term has also appeared in some technical papers describing stegosaurs and related dinosaurs.

The four spikes in the tail of the stegosaurus are now thought to have protruded horizontally rather than vertically.