If you defeat a boss by deleting its stamina gauge or other tricks, you'll unlock bonus camos with unique abilities.

Unlockable	How to Unlock
Animals Camo	Beat Ocelot by reducing his stamina gauge to zero.
Cold War Camo	Defeat Volgin by reducing his stamina gauge to zero.
Fire Camo	Defeat The Fury by reducing his stamina gauge to zero.
Hornet Stripe Camo	Defeat The Pain by reducing his stamina gauge to zero.
Moss Camo	Sneak up on The End and hold him up with a weapon.
Snake Camo	Defeat The Boss by reducing her stamina gauge to zero.
Spider Camo	Defeat The Fear by reducing his stamina gauge to zero.
Spirit Camo	Defeat The Sorrow by reaching the end of the river.