Peter is an enemy featured in The House of the Dead 2 seen in the fifth chapter. His appearance is similar to other undead beings but has a secret way of attack. When an attacker harms his chest, he will be in pain causing a worm to burst out it's chest fracturing his ribcage killing him.
Perhaps during the events, a human must have been fused with a parasite carrying the formula mutating the poor soul. Like any other undead being, siming for the head is safe way in killing him. If shot at the chest, the parasitic worm will burst out but still can be killed.