M'Ress and her tail
The tail was the most posterior anatomical appendage of some animals. The wagging of a tail could indicate the emotion of pleasure. (TOS: "The Way to Eden")
Tailed intelligent species included Caitians, Vedala, and Delta Theta III aborigines. Gorn hatchlings possessed prehensile tails, though adults generally did not.
In an ancient Sumerian myth, Gilgamesh and Enkidu fought the Great Bull of Heaven. During the fight, after Enkidu grabbed hold of the tail, his friend Gilgamesh slayed the beast with his sword. (TNG: "Darmok")
In 2267, while under the influence of the Omicron spores, Spock believed a cloud resembled a dragon, and pointed out the tail and dorsal spines to Leila Kalomi. (TOS: "This Side of Paradise")