From The TimeSplitters Wiki

Hybrid Mutant
Unlocked by	Beat 1990 Siberia on Normal/Hard
Species	Mutant
Size	Normal
Accuracy	8
Agility	3
Stamina	9
Fire Proof	n/a
Shock Proof	n/a
Default AI	5
Native TimeZone	1990
First Appearance	TS2 Story Mode
Relations	TimeSplitters, The Freak
Games	TS2

TS1 Gesture	N/A	TSFP Gesture	N/A
TS2 Gallery	The TimeSplitters evil aura had a devastating effect on some of the more susceptible Siberian troopers, causing massive genetic mutation. Military scientists tried to turn them into super-soldiers, but after a series of horrific incidents the mutants were deemed uncontrollable and placed under maximum security.	TSFP Gallery	N/A

The Hybrid Mutant is a mutated individual found in 1990 Siberia. They are highly dangerous, and are the unlockables of the level.

Description

The Hybrid Mutant resembles a mutated human, with its retarded face and oversized chest, along with a piece of meat connecting its elbow with its body. This is very likely because the Hybrid Mutant is a mutated human. A very ugly character, and a rather freaky one. In Story mode, they are very hard to kill, however 3-4 grenades from the Soviet S47 should suffice.
Apperantly, the TimeSplitters released some kind of virus into these poor soldiers that turned them half human and half TimeSplitter. That is also probably the reason why they have a striking resemblence to Freaks, who are also half human and half TimeSplitter.
Because of the large amount of Zombies in Siberia, it can be assumed that Hybrid Mutants pocess some kind of mutation that zombifies weaker people.