I have seen slaves on horseback, while princes go on foot like slaves.
The scope of these verses is to keep subjects loyal and dutiful to the government. In Solomon's reign the people were very rich, and lived in prosperity, which perhaps made them proud and petulant, and when the taxes were high, though they had enough to pay them with, it is probable that many conducted themselves insolently towards the government and threatened to rebel.
Commenting on Ecclesiastes 10:4-11
I have seen servants upon horses,.... Which being scarce in Judea, were only rode upon by princes and great personages, or such as were in affluent circumstances; and therefore it was an unusual and disagreeable sight to see servants upon them, which was a token of their being advanced upon the ruin and destruction of their masters; a reigning servant is not only uncomely, but...
servants upon horses--the worthless exalted to dignity (Jer 17:25); and vice versa (Sa2 15:30).