Undergrowth

 is a plant ghost monster and the bad guy who appears in Season 3 of Danny Phantom. He has been voiced by Mark Hamill (the actor who played Luke Skywalker on "Star Wars" and has voiced Joker in animated Batman shows). In his debut episode "Urban Jungle", he took over Amity Park and the whole planet using plant manipulation, possesses every person using mind-controlled vines (whatever they're called), and brainwashes Sam Manson to become his wife, the care-taker of all green. His weaknesses include Danny's ice powers and most likely fire. He also appeared two more times: a cameo in "Boxed-Up Fury" and the last episode "Phantom Planet".

Outside the Danny Phantom Universe
After the cancellation of Danny Phantom after 53 episodes, Undergrowth would make his next appearance as a background monster during the Fred the Ostrich generation. In his first appearance on The Fred the Ostrich Show (the George Burns episode), he did the animated remake of the Muppet Meeting Film classic, "Monster and Machine" (or more appropriately, "The Computer Dinner"), taking the role of a monster and voiced by Jeff Bennett. A decade earlier, a Larry the Dairy Monster prototype with teeth appeared in the Dell meeting film "Coffee Break Machine" and he (or it, if no gender) appeared in "Saturday Night Live" when the Fred the Ostrich characters had guest starred. A decade after, Undergrowth redid the remade version of "Coffee Break Machine" where the monster responds by eating a coffee machine. He later made several background appearances of the show, and he made cameos in a trilogy of Fred the Ostrich films The Fred the Ostrich Movie (in the "Rainbow Connection" finale with countless characters), Fred the Ostrich in The Great Diamond Caper (as one of the captive monsters during "Hey, A Movie!"), and Fred the Ostrich & His Friends Take Manhattan (during the wedding scene). Hamill would reprise the role of Undergrowth (in his portrayal as the evil genie) in the story of "Sinbad the Sailor" in the Marty Feldman episode during the show's final year.

Before Danny Phantom
Before he appeared as a one-shot villain on "Danny Phantom", Undergrowth started out as an unnamed green birdlike monster during their pre-Fred the Ostrich appearances on "Saturday Night Live". He also appeared as one of the Creeps in the 2000 holiday TV special Fred the Ostrich Saves Christmas, and he made several times in a pilot for The Fred the Ostrich Show called The Fred the Ostrich Show: Sex and Violence, where he dances with a red beaked woman during "At the Dance".