When a man without money grows suddenly rich, he is liable to become the most
arrogant of mortals. John Wolcot, whose pen-name was Peter Pindar, wrote in
his _Epistle to Lord Londsdale_ :

> Such is the bad effect of wealth — rank pride —
> Mount but a beggar, how the rogue will ride!

The term ‘beggar on horseback’ is used to describe an upstart, one who has
suddenly gone up in the world and has no manners.