That's the Spirit is the Halloween special of the TV series of Phineas and Ferb.
Plot
Phineas, Ferb, and everyone else are trick-or-treating. They come across a young boy named Russell, who claims that his house is haunted. They decide to investigate, and meet many surprises inside. Meanwhile, Doofenshmirtz, due to a hatred of grass, has switched brains with a cow, turning him into a Werecow whenever there is a full moon.