From The Grim Adventures of Billy & Mandy Season 2 Episode 6b: "Circus of Fear"
A potential reference to Earthworm Jim 2, where Princess What's Her Name, Psy-Crow, and Earthworm Jim are all revealed to be cows in the end.
Clortho: Put that down, now!
Mandy: I don't think so. [whips Clortho's mask off to reveal Dr. Fear]
Grim: Clortho was Dr. Fear all along?
Mandy: Not exactly. [whips Dr. Fear's mask off to reveal Gladys]
Billy: [concerned] My Mom's Dr. Fear?
[Mandy whips Gladys' mask off to reveal Eris]
Billy: [confused] Eris is Dr. Fear's Mom?
Mandy: No, Billy. Dr. Fear is— [whips Eris' mask to reveal an earthworm] —a common earthworm.
Grim: Just as I suspected.
Billy: I don't get it. How the heck can he be an earthworm?
Mandy: Billy, if you paid attention at all, it would be obvious that he'd been an earthworm all along.
Billy: But I—
Grim: She's right, Billy. You're just dumb.
