Boundless said:

Alright, this is kinda simplified, but:

In the wild, or even turned out in pastures, horses live in a fission-fusion society (kinda like elephants). A few mares and one stallion live as a family unit in a small band. All yearlings are generally chased off, or leave of their own accord, so as to avoid incest and expand the gene pool. Bands will often run into each other, and the horses will often know one another (hence the larger society, even though there are smaller harems/band subgroups). A stallion might try to overtake the other, etc., it depends.

So, there really isn't a lead mare. The stallion tends to be the only one to 'herd' the band (exactly what it sounds like - moving the horses from behind), but mares also exhibit this behavior - it really depends form horse to horse, but usually, it's the stallion. Each horse can be more dominant in their own respect - for example, one can be more territorial over water, while another shade, and another food. The older the horse is, the higher ranking it tends to be. There are certainly horses lower in the hierarchy, and those higher, but it isn't always as clear-cut.

So, usually, mares will follow any mare. This is called departure behavior - if one mare leaves/walks away, the others are likely to follow. The higher ranking the mare is, the more likely the others are to follow, but horses will also follow the lower ranking as well. Generally, where the herd moves to and goes is somewhat of a group decision - it's not lead solely by the stallion, nor a certain mare, most often.

That's a simplified run down, basically!