The Hungarian Horntail.
GoF
The three schools involved in the 1994-95 Triwizard tournament.
Hogwarts, Beaubattons, and Durmstrang
The singers of Come Seek Us Where Our Voices Sound.
The Merfolk/The Golden Egg
Vernon Dursley
The Leader of the Slug Club, whom it is named after.
Horace Slughorn
Aunt Marge's Big Mistake
PoA
The three Hogwarts students held captive by the merfolk during the second task.
Ron Weasley, Hermoine Granger, and Cho Chang
The Shakespearian play in which the song Double Trouble, featured in the PoA, gets it's lyrics.
MacBeth
The American term for Muggle.
No-Maj
The location of first 1996 Slug Club meeting.
The Hogwarts Express
The Midnight Duel.
PS
The three school's headmasters.
Albus Dumbledore, Olympe Maxine, and Igor Karkaroff
The song in which these lyrics feature: He cannot block a single ring/That's why Slytherins all sing.
Weasley is Our King.
Lord Voldemort's Muggle father.
Tom Riddle Sr.
The member of the slug club who gave Professor Slughorn a goldfish.
Lily Evans (Potter)
Owl Post Again.
PoA
The three dragons not faced by Harry during the first task.
The Chinese Fireball, the Swedish Short-Snout, and the Common Welsh Green
Molly Weasley's favourite singer.
Celestina Warbeck
The Muggle Studies teacher at Hogwarts prior to her 1997 murder.
Charity Burbidge
The impressive hex that Ginny Weasley performed, which got her invited to join the Slug Club.
The Bat-Bogey Hex
DH
November, February, and June
The song Harry and Hermoine dance to after Ron's departure in DH1.
O Children
Kannnichtzaubern
Cormac McLaggen's well-connected uncle.
Tiberius