A haggis, (according to Burns, "the chieftain of the pudding-race,") is an olio, composed of the liver, heart, &c. of a sheep, minced down with oatmeal, onions, and spices, and boiled in the stomach of the animal, by way of bag. When the bag is cut, the contents, (if this savoury dish be well made) should spout out with the heated air. This will explain the allusion.
A Muffled Man means a person in disguise; a very necessary precaution for the guide's safety; for, could the outlaws have learned who played them this trick, beyond all doubt it must have cost him dear.
From this it would appear, that Carey, although his constant attendants in his fort consisted only of 200 horse, had, upon this occasion by the assistance, probably, of the English and Scottish royal garrisons, collected a much greater force.