Why does this 757 have a propeller engine attached to the fuselage? I saw the following photo of a Honeywell Aviation Services Boeing 757-225:



If it's not a fake, the airplane has a propeller mounted on a pylon on one side.

What's the purpose of such strange design? Why does it need a propeller engine, if it already has two jet engines? Or is it a permanently deployed RAT?
From the description below the image you linked:


  The Honeywell, Boeing 757 test bed appeared on static display for a few days before departing half way through the event.


A testbed aircraft is an aircraft used for flight research or testing new equipment such as engines.

You can see other photos without engine or with a jet engine instead of a propeller one.

The engine is not needed to fly, but is being tested, and apparently that position has been deemed a good solution to the problem "where do we attach an extra engine we need to do tests on?".