Samuel Johnson referred, disparagingly, to this in his dictionary definition for oats: "A grain, which in England is generally given to horses, but in Scotland supports the people." His biographer, James Boswell, noted that Lord Elibank was said by Sir Walter Scott to have retorted, "Yes, and where else will you see such horses and such men?"

Tomorrow is the 300th anniversary of the birth of Samuel Johnson, a man who, by his own account, stank. The above quotation is lifted from wikipedia. Johnson's crack about oats and the Scots is often quoted; the noble lord's retort tends to be ignored, as it takes the shine off Johnson's putdown. Several remarks by Johnson antipathetic to the Scots are on record.
One might reply to Elibank's query with the single word "Ireland". After all, that nation's bloodstock is the most highly rated, and it has furnished more canon fodder for empire builders than even the Scots. But leave that aside, and savour a remark that deflates prejudice, albeit witty, with further wit. It has to be stated in Johnson's favour that he confessed that he always quitted Elibank's company a wiser man.
Johnson's stupid claim that the Highland Gaels had no real language, but communicated by means of a series of grunts, rather places his much vaunted erudition in perspective. That particular example of his ignorance served him in good stead when he challenged James MacPherson to produce the original copy of his Ossian "translations". Johnson was right, there was no original, the "translations" were MacPherson's own work. But his challenge was based on his own ignorance. He believed that no book had ever been written in the Gaelic language.
The Scottish Gaeltacht* held plenty of manuscripts, produced by learned families, though few were in bound book form. However such books did exist, so it was only a matter of luck that Johnson was able to succeed with his challenge based on a misconception.

Of the two frauds my preference is for "Ossian" MacPherson over the smelly doctor.

*Or "Gaidhealtachd"