Well as difficult as the Patronus Charm is Harry did teach at least the rudimentary spell to students of various years in the Room of Requirement

[Hider=Dumbledor's Army]

Dumbledore's Army Parchment Signatures

Alice Tolipan
Alicia Spinnet
Angelina Johnson
Anthony Goldstein
Cho Chang
Colin Creevey
Cormac McLaggen
Dean Thomas
Dennis Creevey
Ernie Macmillan
Fred Weasley
George Weasley
Ginevra Weasley
Hannah Abbott
Harry Potter
Hermione Granger
Justin Finch-Fletchley
Katie Bell
Lavender Brown
Lee Jordan
Luca Caruso
Luna Lovegood
Marietta Edgecombe
Michael Corner
Neville Longbottom
Nigel Wolpert
Padma Patil
Parvati Patil
Romilda Vane
Ronald Weasley
Susan Bones
Terry Boot
Zacharias Smith[/Hider]

As for Miss Night's Patronus I can only say that hers fits her interest in the dead and the afterlife. She's also Greek and conected to the temple of Hecate a goddess know to be connected in mythology to  ghosts, necromancy, and sorcery.

Cerberus guards the entrance of the Greek underworld to prevent the dead from escaping and the living from entering.

These are reasons that caused me to chose Cerberus. [@Tatsua Aiisen]