The Dregs Theory 

“Cock tailings” was a term used to describe the mixture that was created when tavern owners combined the dregs (tailings) of nearly empty barrels together. The resulting mixture from several barrels would then be sold at a discounted price. This only makes sense when you know that the spigot of a barrel was sometimes referred to as a “cock.”

The Egg Cup Theory 

According to the online Etymology Dictionary, the origin of the cocktail is  down to a mispronunciation of the French word for eggcup coquetier, pronounced in English as cocktay. Apparently, a New Orleans apothecary, Antoine Amédée Peychaud, served brandy with his bitters in eggcups in the late 18th century.

The Docked Horse Theory 

In the 17th Century the term “cock tail” was used to describe an animal with a cock-like tail, specifically for horses with docked tails. Coach horses and hunt horses tails were often docked for practical purposes. By the 19th century, unlike other horses, thoroughbreds did not have docked tails, so when a regular horse was entered into a race, its cock tail was noted.