Why is the horizontal stabilizer forward of the vertical stabilizer on the P-51?
(Source) Corsair top, P-51 bottom.

The noticeable difference above is the taller rudder of the P-51.

A taller rudder is needed for a shorter lever arm (distance between empennage and wing) and/or the need for more yaw authority.

And as @Gerry pointed out, another benefit of having part of the rudder below the elevator line is for spin recovery when the horizontal stabilizer is stalled and sending its wake upwards. The alternative is like the Corsair, with the rudder ahead and clear of the horizontal stabilizer.

Is it unique? No, a Cessna is shown below:





Related: How many versions of the P-51 dorsal fin fillet are documented?