Alcohol
Alcohol is a type of beverage typically reserved for adults.
Behind the scenes
The only known alcoholic beverage to mentioned in My Little Pony fiction is Dom Perington which was mentioned in "The Stage".
References to alcohol
Apple Cider is typically used as the stand-in for alcohol in My Little Pony: Friendship is Magic. Characters who drink it typically get stereotypical "drunk" faces and in real life, cider is typically an alcoholic beverage, but non-alcoholic cider isn't exactly rare either. One of the more blatant examples was in the manga story, "Are They Rivals!?" where Rainbow Dash glugged down an entire cup of the substance after being tricked by Applejack.
In the "Adventure!" arc, Truth Serum, a type of juice made from Horseshoe Bay Pineapples was a parody to alcohol.