The Demon Beast Warriors of Emperor Ganishka's Troops. These various animals (to date: Crocodiles, Elephants, Tigers and Whales) are possessed by Kushan magic (implanting a spirit into an animal) and sometimes outfitted with human weapons. The nature of the possession requires a Kushan casters to locally control the minds of the animal (in doing so they must be in a state of meditation). Emperor Ganishka uses this inhuman force to dominate Wyndham and invade Vritanis. They can still be affected by the barrier that rejects astral bodies though not entirely since they have an actual body (originally animals) unlike Trolls that came from the astral world who are pure spirits.
When their possessors are killed their minds go back to their animalistic nature making them less dangerous though their monstrous appearance and possibly their enhanced abilities (since crocodiles can still carry their spears) aren't reverted.
Once dead Pishachas don't revert to their original form immediately, as it will take some time.
The term Pishacha means devils in Sanskrit. An apt description for their fighting style and abilities.