Film: Harry Potter And The Order Of The Phoenix
When the aggravatingly-prim Dolores Umbridge started her tenure at Hogwarts she crippled the school by banning practical magic and teaching students nothing other than textbook theory. To counter this situation, Ron, Harry and Hermione create Dumbledores Army also known as the D.A. or Defence Association to give their peers some helpful advice in Defence Against the Dark Arts.
At first it seems like the secret organisation is doomed to fail as the first D.A. meeting held in the Hogs Head seems to be completely unattended. The trio walk into the bleak pub and find it deserted other than a barman and a curiously random goat behind the bar. It turns out that this goat is actually an Easter Egg referring to Aberforth Dumbledore who we encounter in the penultimate film. The middle sibling of Albus and Ariana is described as having some sort of obsession with goats which rather worryingly gets him in trouble with the wizarding authorities at one point.
The seemingly out-of-place goat is a subtle Easter Egg, which announces that the dilapidated establishment is owned by Albus younger brother. Some believe the barman to be Aberforth himself, though when we see him in the later title he is played by Ciarán Hinds.