Un potro is a colt.  It is a young horse no older than 4 years.   La carne de potro is the meat of a colt.  It is considered to be very nutritious and low in fat.  In Spain, this meat can be served in many different ways, just like other meats, for example:  as a roast dinner, in stews, as a steak, in sausages and burgers. 

Una hamburguesa de potro is a burger prepared with this type of horse meat.