The shapeshifting alien bounty hunters all appear to look a bit like Arnold Schwarznegger as the Terminator. This is their default setting, but, as the name 'shapeshifter' suggests, they can look like anything. It seems they get their shapeshifting abilities from the Black Oil.

Referred to as 'alien bounty hunters' by Chris Carter, these shapeshifters act as policemen for the colonising aliens. Their tasks are to clear away the evidence of alien abductions, to eliminate resistance, and to destroy the rebel alien clones.