Years ago, ponies and human began living alongside one another. Some humans living in Equestria, and some ponies living on Earth. As ties between the two species grew, so too did the frequency of relationships between them, which, of course, led to more intimate goings on than either side could have predicted. And given certain special times of the year for mares, this became something they were more than willing to go along with.
These are the many, many tales of the times when ponies and humans get a little closer than normal.