Lawn gnomes

A Lawn Gnome or Garden Gnome is a figurine of a small humanoid creature, usually wearing a  pointy hat, displayed for the purpose of ornamentation on  front lawns or in  gardens. These figurines originated in 19th-century Germany, where they became known as  Gartenzwerge (garden  dwarfs). The application of the term gnome in English is first attested in the 1930s.

Doof's flashbacks
During his childhood, back in Gimmelshtump, Heinz Doofenshmirtz was forced to be a Lawn Gnome by his parents.

2nd Dimension
Heinz Doofenshmirtz-2 had an army of Lawn Gnomes for singing his entry tune

Appearences

 * "Lawn Gnome Beach Party of Terror"
 * "Got Game"
 * "The Chronicles of Meap"
 * "Rollercoaster: The Musical!"
 * "Phineas and Ferb The Movie:Across The 2nd Dimension"
 * "Meatloaf Surprise"
 * "Father's Day"