Monster: Genetically engineered pony
Appearance: "Dee-Dee's Pony Tale" Dexter's Laboratory #16. December 2000.
John Delaney: Penciller, Jeff Albrecht: Inker, Ryan Cline: Letterer, Lee Loughridge: Colorist, Harvey Richards: Assistant Editor, Joan Hilty: Editor. Dexter's Laboratory created by Genndy Tartakovsky
For Dee Dee's birthday, Mandark decides to create for her the perfect pony using horse dna, dna from a giant squid to give it big eyes, dna from the pinkest earthworm, the yellowest canary, and the silkiest silk moth. This results in the pony above.
Of course this story required some science fictional comedy, so the horse doesn't remain stable, and ala John Carpenter's The Thing, transforms into a monstrous specimen, which Dee Dee is still delighted with and manages to tame. Will her parents let her keep it? That would be telling.