This is a dig at Scotland, which has been the butt of English humor for most of history. Oatmeal is a perfectly edible food, nutritious and healthy, but it was not popular in Swift's England. In Scotland, which was a much poorer land, oatmeal was a staple. Gulliver the Englishman, unable to eat hay and straw, had to content himself with the food of a Scotsman.
In 1755, Samuel Johnson (1709 - 1784) in his famous dictionary defined "oats" as a grain which, in England, was fed to horses and , in Scotland, was fed to men.
The Houyhnhnms are puzzled by Gulliver. Because of his clothes, his ability to reason and his similarity to Yahoos, he is neither Houyhnhnm nor Yahoo. Because of his mixture of qualities, he is given a place to live that is between the lowly Yahoos and the high Houyhnhnms. Symbolically, Gulliver (man) is midway between the Houyhnhnms (innocence and perfection) and the Yahoos (depravity and chaos) and partakes of the qualities of both.
The Houyhnhnms are portrayed throughout this section as we might imagine Adam and Eve before the Fall. The Yahoos are portrayed as what man might become if there was nothing in him worth redeeming.
It is Gulliver's tragedy that he cannot accept his midway status. He does not understand that to strive to be like the Houyhnhnm is as unsatisfying as it is to plunge to the depths in which a Yahoo exists. It is as great a loss to lack passion, even if there is perfect calm, as it is to lack reason when there is nothing but chaos.
High Dutch is a dialect of German spoken in northern Germany and the Netherlands.
Emperor Charles V (1500 - 1558) was Holy Roman Emperor, King of Spain, and ruler over parts of Italy, the Netherlands and much of North and South America, as well as various islands.
In discussing the chief languages of his dominions, he supposedly said that he would "speak Spanish to God, Italian to women, French to men, and German to my horse." This was an expression of his contempt for German, which he did not speak well.
Prince of Orange is a title of nobility, originally associated with the Principality of Orange, in what is now southern France.
It is carried by members of the House of Orange-Nassau, as heirs to the crown of the Netherlands, and is also seen carried by the pretenders by members of the House of Hohenzollern.
James II of England, who had come to the throne in 1685, was a Catholic, while most of the nation was Protestant. He was a poor politician and offended the leaders of the nation. When he married again and had a son in 1688, those leaders planned to remove him from the throne in favor of his older daughter, Mary, by a previous marriage. She had been raised by a Prostestant. She was married to her first cousin, William of Orange, stadtholder of the Netherlands, and also a Prostestant.
The plot was successful. James II was forced to flee and was succeeded by his nephew and his daughter, who ruled jointly as William III and Mary II. The changeover was virtually bloodless and was therefore called "the Glorious Revolution." It was accompanied by the acceptance by William and Mary of "the Declaration of Rights" on February 13, 1689. This made Parliament supreme in the nation.