'Rabbit or Duck' - Season 5, Episode 15
Ted is set up with one of Barney's discards by Marshall and Lily. Meanwhile, Don invites Robin to a party - which she suspects is really a date.
Air Date: February 8th 2010.
Barney: Keep your eyes peeled for a red sweater. Based on her texts, she's dirty, dyslexic, and wants to 96 me.
Ted: Why don't we just get a rabbit and a duck, put them in a fighting ring, and let them fight it out!
Marshall: Because that's illegal, Ted!
Ted: Only if we bet on it, Marshall!
Robin: So that settles it. Don's a rabbit.
Ted: I wouldn't be too sure. Don ... Donald ... Donald Duck? And what does Donald Duck never wear, I wonder? Pants.
Ted: Don's a duck. Requesting permission to lawyer.
Marshall: I'll allow it.