I just quit polo; I feel bad for doing it but became a vegan. Right or wrong?

Be careful. You will die miserably and slowly if you consume only foods of vegetable origin without supplementing. If you feel morally superior or physically more vigorous as a vegan, do it -- but you will need to supplement. Without carnivores, the surface of the earth would be nothing but a layer of bacteria -- yet I am sickened by the way factory-farmed animals are treated. It's your choice.
Depends. I am assuming you became a vegan for philosophical reasons along with health benefits. This would include, therefore, animal exploitation which the extreme vegans incorporate in their lifestyle. Polo horses are generally well cared for and seem to enjoy their lifestyle. But if you feel that their use is exploitative, you are within your rights. There is no absolute right or wrong for this.