Is there any way that a pet immune motion sensor is available for X10 security systems? ...Does anything like that exist with X10 ?
No.
But, following along with the lines of
steven r's reply:
The only way I know to keep a sensor from detecting pets is to adjust it for a zone above them.
you can
experiment with the plastic lens.
All of the X-10 Motion Sensors lenses, AFAIK, as delivered, point slightly down - check the manuals for actual coverage diagrams. Each rectangle on the lens is a "zone" - a 'finger', a 'spoke of a wheel' - reaching out.
You could try inverting the lens (so that it would point slightly upward), thus ignoring motion below the "zones". Or, you could experiment with Liquid Paper and/or small pieces of black tape and selectively BLOCK off certain "zones" until you achieve the result you desire.
Good Luck!