Summary: Evolution of tail weaponry (found in ankylosaurs, stegosaurs, and some sauropods, among dinosaurs) is relatively rare among animals and appears to be strongly correlated to anti-predator defense. It is possible in glyptodonts (armored mammals related armadillos) and ankylosaurs may have had sexually-selected weapons due to their size and complexity, it seems unlikely that this was the case in all other animals. Tail defenses seem to follow selection for body armor and a rigid thorax, and seems to occur in medium to large herbivores. Tail defenses seem to be a last resort defense when other defensive strategies (crypsis, escape,) have failed. 

[Article] Evolution of Tail Weapons in Amniotes