The Finisher was the 5th opponent chicken Jack faced in the Creature Combat Club.
It's a large rat that is shown to be both filthy and hideous. The rat usually preforms a noise that it makes with its tongue which sounds like "Leh!". It is also actually mechanical, as shown when Jack slays it with the Mutant Earthworm's acidic blood.
The Finisher was meant to beat Jack when he was injured by Mutant Earthworm. When it came out of its dungeon-like container, it was booed because there was already an opponent fighting and releasing The Finisher was cheating.
Fleas: The Finisher scratches himself with his legs and arms launching dirt and fleas on the opponent.Pounce: The Finisher launches itself at the opponent, trying to bite into them.