In the episode called "The Day the Earth Stood Stupid", where the giant brains make everyone dumb, Nibbler goes back to his home planet with Leela. If you look at the female Nibbloneon of the group, you will notice she has no fangs. When they ask Leela to go back to earth and give Fry the message, the female Nibbloneon tells Leela that when back on Earth, Leela will be too stupid to remember the message. If you look at her mouth when she says that, you will notice she now has fangs! Either someone's made a mistake or they are false teeth she's wearing!