How could the Mongols get milk from their mares if they werent with foal?

It's very difficult, but possible to lactate without pregnancy/birth. It's sometimes called "Induced Lactation". One of my dogs (never bred) produced milk and raised two kittens. I've heard of numerous other cases, involving various animals. I've even heard of adoptive human mothers nursing their adoptees. Birth or not, milk is supply and demand. There's no certain time it 'dries up'. I hope that helps.