I have examined every published history of the unit to appear in the last 170 years and have discussed the matter with the RWF themselves but they have no accurate information beyond a statement in Donkin's book that, when the 23rd was in Boston in 1775, they had the Goat with them. Donkin remarks that keeping a Goat was already a longstanding tradition in the unit and popular belief in the RWF is that the tradition started sometime in Marborough's time.
If anyone is surfing through pre-1775 military literature or other sources and comes across a reference to the 23rd Foot and the Goat, please let me know.
In Boston in 1775, the Goat misbehaved during the annual St. David's Day celebration dinner, threw off the drummer boy riding on its back, bounded across the mess table, scattering food and drink in all directions and ran out the door into the streets pursued by most of the battalion, much to the amuseument of the colonials who, pausing from their favourite activity of huddling in dark corners to plot treason against their Sovereign, watched the chase with interest.
Coincidentally, to mark their joint participation in the Peking Relief expeditionn of 1900 the RWF presented the USMC with a goat which was the marine mascot until 1919 when the corps adopted the bulldog (a more tractable animal). The marines gave their goat to the USN and a goat is to this day the mascot of the American navy.
DE Graves