I find it hard to imagine anyone who can afford an aeroplane big enough take
their horse in would be still serving
The Sentinel was designed to fit the officer's horse - you can see the bulge for its head on the top and the trough for its feet under the fuselage.
Did you know you can claim MMA for taking your own aircraft on duty
journeys?
I knew a female engineering officer of significant "private means" who had her own private twin (Queenair?). She regularly had battles with PSF when claiming duty journeys that she had taken in it. She always won by quoting the appropriate QR.