Neville had yet to cast a full Patronus, and he was admittedly as nervous about what form it would take as he was what it would protect him against. Your Patronus said a lot about you, and with Neville's luck, his would be something like a snail, or a turtle that refused to come out of its shell.
"Thanks to Professor Lupin I'll always think of a Boggart that's Professor Snape in my gran's clothes," Neville chuckled softly, looking about surreptitiously to be sure the erstwhile Professor was nowhere around. It was amazing, how he managed to stumble upon Neville whenever the boy was about to do something he could deride him for.
Of course, Neville did a lot of things he could be teased for, so it wasn't that much of a surprise.