Monster is a minor character in Mortal Kombat: Deception. Shujinko encounters him during Konquest Mode inside the Nexus, where Monster claims to be the true champion of the Elder Gods, and challenges Shujinko to Mortal Kombat. After he is defeated, he is not seen again.
Monster was originally going to be an alternate costume for Scorpion. It was his rumored "Knight" costume. The developers felt that it didn't fit Scorpion, and that idea was scrapped.
Konquest was originally supposed to involve time travel, thus explaining Monster's (Scorpion's) appearance before his death, the fact that he is the true champion of the Elder Gods, and most likely other appearances/events that happen out of sync with the MK timeline. The developers thought the time travel idea would be too confusing, and it was scrapped. However, they liked Monster so much, that they left him in. He retained all of Scorpion's moves, and the "true champion of the Elder Gods" story. They never fixed the timeline after scrapping time travel, thus leaving a lot of confusion amongst fans as to whether or not Monster is to be taken as a new character, or as Scorpion.
Monster is playable via an Action Replay code, which if you activate the code, will make him replace Scorpion.