Set a begger on horseback and he’ll ride to the devil

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.