Thanks for the corrections Wadmalatz and PostmasterGS.
I should have known this as I do have Section 2 of H&G and I've seen them before.

The Saxon horse emblem was used by a number of German states and families. This included the British Kings when they ruled both Hanover and Britain and the Dukes of Brunswick were from another branch of the same family.
Here for comparison is the flag of the Duchy of Brunswick:

