The imaginary character named "Koos" mentioned and briefly shown in Cootie Gras is a character from Dexter's Laboratory (1996).
The voice Tara Strong uses for the Imaginary Rabbit Girl is the same used for Twilight Sparkle in My Little Pony: Friendship Is Magic (2010).
Although this episode premiered in 2000, it was produced in 1999 according to the credits.