Forlong the Fat and his horse, because the latter would only consent to carry him at times of great need, ie war?
But seriously, maybe a soldier of Gondor (Stoningland) who normally just manned the citadel, riding a horse (possibly of Rohan) during the war?
Beregond on a steed of no particular name? I had thought for a moment he was not at the Last Battle because he was being held pending trial, but of course he was - he was with Pippin and was overborne by the troll. And after the war he went to serve Faramir in Ithilien.