The Gelatin Monster was created by Phineas and Ferb, and first appeared in "Day of the Living Gelatin." It was first part of Phineas and Ferb's project as a giant batch of grape-flavored gelatin in Isabella's pool, but when Doofenshmirtz's Turn-Everything-Evil-inator hits it, it becomes a monster. The kids believe the source of the evil is Baljeet's curry gelatin flavor, to which he retorts that it is only spicy. It eats Candace, and later her sculpture of Jeremy's head during Come On, Kids!.
The kids attempt to destroy the gelatin monster by spraying it with water guns, but Doofenshmirtz calls it and the monster escapes to Doofenshmirtz Evil Inc. by "melting" through Isabella's pool drain. Doofenshmirtz commands it to defeat Perry the Platypus, but Perry turns the fire sprinklers on and dissolves the Gelatin Monster. Doofenshmirtz cries as the monster is melted.
It seems to be able to split itself into two or more of itself, as shown in the song "Come On, Kids!".