Kramer makes up this name to sneak into his mother's ex-boyfriend's apartment to reclaim a jacket.
(Episode #26)The Nose Job
Elaine: Won't the landlord know I'm not the daughter?
Kramer: No no, he's never met her. She's in California.
Elaine: Are you coming with me?
Kramer: Oh, yeah yeah, I have to. I'm your fiancÚ, Peter Von Nostrand.
George: Why don't you just commit yourself already?
Audrey: What is so special about this jacket?
Elaine: He believes it possesses some extraordinary power over women.