You start off with 2000 Equus (e). To see the history of your equus, click on your balance and scroll down. There you will see your financial history. To see, more click on the "See complete history" button.
Sometimes you can win equus from promos, like the Lottery.
To get equus for your EC (Equestrian Center), you can sell items to the shop, credit your reserve with passes and diamonds, and use the equus your boarders pay. You get 25,000 equus when you open your EC.