Gudon is a kaiju that has appeared as an antagonist in the Ultraman franchise, first appearing in episodes 5 and 6 of Ultraman Jack.
Return of Ultraman
Long ago, the Gudon species was the predator of the Twin Tail species. In the present day, a Gudon emerged after MAT unearthed a Twin Tail egg to do battle with its ancient rival once more.
After killing Twin Tail, Gudon turned its attention to Ultraman Jack, though it was unable to hold its own against Ultra, who overpowered it and destroyed it with the Specium Ray.
A Gudon appeared in episode 2 where it caused havoc at a construction site. Ultraman Mebius soon arrived to fight the monster and defeated it by slicing it with the Mebium Blade.
It reappears in episode 27. While GUYS were testing out Capsule Kaiju, a Gudon was sent out to fight Zetton, only to be defeated and retreat back into its ball.
Ultra Galaxy Mega Monster Battle
A Gudon appears in episode three where it fights a Neronga, only to be defeated and killed by it.
Mega Monster Battle: Ultra Galaxy Legends The Movie
Gudon appeared as a member of the Belial Army. It took on Ultraman alongside Antlar, Alien Baltan, Gomess, King Gesura, Arigera, Twin Tail, Goldras, Magular, Silvergon and Zetton. It later fought Ultraman Zero when he arrived and was destroyed by Zero's Wide Zero Shot along with Alien Temperor, Telesdon and Vakishim.
Gudon appeared as one of the many kaiju cheering on Ultraman Ginga as he fought Dark Galberos.