It is, isn't it. But now I realize I've never said it out loud -- just read it! So how does one pronounce leonine? Sort of like I never knew how to say Hermione until I heard it in the movie. I think I was saying it, in my head, Herm-E-on. Ooops. :)
I believe it's Lee-on-nine (though I've heard some people say neen). My friend's dad read HP aloud to her and to this day she finds it weird hearing people pronouncing Slytherin as 'slither-in' because apparently he said it very differently!