Hmmmmm, this is giving me an idea.....
Say there is a race of psychically blunt aliens that occasionally come into contact with the Inquisition without the Inquisition realising (they watch inquisitors fight battles against chaos on their world while hiding, etc).
On one of these occasions an inquisitor brings along a psyker. They see the psyker's powers in action, and assume that anything carrying that kind of awesome, unnatural power must be a god or a great servant of a god.
On another occasion, they see a culexus assassin destroy a chaos sorcerer, being seemingly impervious to the sorcerer's god-like abilities. They assume that the culexus and others like it are servants of another god.
This leads the aliens to begin worship of the human form as vessels for gods, and in particular psykers and untouchables. When a psychically active, radical inquisitor stumbles across this species, they begin to worship him. Seeing the usefulness of having an entire species under his command, the inquisitor indoctrinates them into a slightly modified version of the Imperial Creed. The aliens then begin worshipping the Emperor as a god, and have a great respect and fear of untouchables, seeing them as servants of the opposite god to the Emperor. The inquisitor then goes to great lengths to hide the existence of this species.
Now, whenever he needs a cheap meatshield or an agent with inhuman abilities, he calls upon members of that species, saying that the Emperor has chosen them to do His work. The aliens are then used to carry out his dirty work for him.
The version of the Imperial Creed the inquisitor taught the aliens contains the idea that if one fails in the Emperor's work, one does not deserve to live. This gives the Inquisitor a failsafe if any of his alien retinue are captured, as they will think they have failed the Emperor and will so commit suicide.
I'm thinking lots of aquilas, purity seals and possibly augmetics in a human-like form, as the aliens would aspire to become like humans.