'I'm supporting Gryffindor . . . It's good, isn't it? . . . I wanted to have it chewing up a serpent to represent Slytherin, you know, but there wasn't time. Anyway ... good luck, Ronald!'
-- Luna Lovegood (OP19)
For the first Quidditch match in Harry’s fifth year, Luna Lovegood, a Ravenclaw student, supported the Gryffindor Quidditch Team by wearing a hat that resembled the head of a full-grown lion (OP19). It could roar so loudly that it made people jump in the Great Hall, but that also meant Harry could hear it over the sound of the crowd, which encouraged him during the match. Luna originally wanted the lion’s head to be chewing up a snake to represent victory over Slytherin (OP19).
Harry can hear Luna’s hat roaring in the crowd when the Gryffindor team again play against Slytherin during the following school year (HBP14).
Actress Evanna Lynch has said more than once that she loved the lion hat in the movies, as in this Universal Orlando interview:
What is your favorite Luna costume or accessory?
Evanna Lynch: The Lion Hat, obviously! That thing was fierce (literally)!
Also this from the Deathly Hallows 2 cast press conference from Digital Spy:
"My favorite thing would be the lion hat because the thing about wearing Luna's clothes is that they're kind of the clothes that no one would wear in their right mind. So you have to -- you get out of yourself, you're very free, and the lion hat was that intensely 'I can't care what anyone thinks of me - I'm wearing a lion on my head.' And it was very light - deceptively light - and comfortable. I kept forgetting I was wearing it and people kept looking at me, like 'what?' - it was really fun, yeah."
In May of the same 1995-96 school year, just before Gryffindor play against Ravenclaw, Hermione and Harry see Luna wearing a flapping "live eagle" hat on her head - although it may be a raven, or should be, to show support her support for Ravenclaw House (OP30).
From the Web
Harry Potter Wiki: Luna Lovegood's lion hat