The Big Bang Theory Season 1 Episode 6: "The Middle-Earth Paradigm"
The guys are invited to Penny's Halloween party and decide to go only when the idea of costumes are mentioned. At the party, Wolowitz and Leonard attempt to socialize until Penny's ex, Kurt, shows up and humiliates Leonard.
Leonard: I understand your impulse to try to physically intimidate me. I mean, you can't compete with me on an intellectual level and so you're driven to animalistic puppery. Kurt: Call me a "puppy animal?"