Betty Lou is a young girl from Sesame Street, and has been variously played over the years by Jim Henson, Frank Oz (in the game show "Pick Your Pet", where she's called Lucy Jones), Carroll Spinney, Fran Brill, Marilyn Sokol(in the songs "The Subway" and "I want a Monster to be my friend"), and Lisa Buckley.
She is made from the Hot Pink Little anything Muppet, much like Prairie Dawn. Prairie and Betty Lou are sometimes difficult to separate. For example, sesamestreet.org once used an image of Prairie for both her character page and Betty Lou's. (As of December 2009, though, the site has replaced the picture with one of Betty Lou.) A 1994 document from the CTW archives also indicates the difference between the girls by their hair.