Well, I'm hoping you'll allow Manawyth a fair defence. Maybe a boring old trial by a jury picked by the Eorl, maybe an ordeal by fire or ordeal by combat...
In terms of punishment,
here is a pertinent link. Open theft of something as valuable as a horse, particularly in Rohan, might well be death or mutilation as a bootless crime...though Gandalf seems to have got away with it, admittedly...