There are numerous undescribed tail clubs of diverse morphologies that may be assigned to Euoplocephalus (Ornithischia, Thyreophora, Ankylosauridae) of middle Campanian to early Maastrichtian age. Among these is an exceptionally small club, the smallest so far described from North America. Most, but not all, clubs can be placed into one of three shape categories: round, bluntly pointed, or elongate. Much of this diversity is ontogenetic or individual, but some of it may be taxonomic. Caudal structure restricts lateral, and especially vertical, tail flexibility. Analysis of hindlimb length, tail length, and downward angle of the tail from the hips suggests that the tail was normally carried and swung just above the ground, and was used primarily defensively, for striking at the metatarsals of an attacking theropod. Intraspecific, agonistic functions are possible, but improbable.