To unlock the Pony Up trophy in Red Dead Redemption 2, you must spend a total of $5,000 in the game's various shops.
Getting the $5,000 might be a struggle during much of the game unless you are using treasure maps to find gold bars or going out of your way to rob and steal everything that moves.
The simplest method is to wait until after you have completed the epilogue as you'll automatically be given $20,000.
Once you have the money, save your game. Head to the stables in Saint-Denis and buy horses as they are the most expensive item in the game.
Once the trophy has unlocked, you can reload your previous save.