In order to fully complete GTA: San Andreas and get that 100% tropy, you're going to have to look for the 50 hidden Horsehoes.
Each Horseshoe found will increase your luck by 20 and net you $100. Oddly enough, luck has no effect on betting, nor on the outcome of gambling. Instead, it's simply an indicator of how many Horseshoes you've found.
With the 50th, you'll have 1000 luck -- but as a bonus, you'll net $100,000, and a weapon will appear in front of each of the four casinos in the game. They are an MP5, a semi-automatic shotgun, an M4, and explosive charges.
How to find Horseshoes in GTA: San Andreas
First of all, be mindful that many Horseshoes will be difficult to reach. They'll typically be at a height, on decorative features or on rooftops. Using a jetpack or a helicopter can make your life a lot easier, allowing you to reach them effortlessly.
The good news is that the fifty Horseshoes are all found in Las Venturas, so you don't have to travel across the entire map to find them.