When you breed two horses, the foal will be born fully grown, but also buggy. It will stand on one spot, just rotates around, cannot be mounted and slides over the ground when pushed or leashed. However, you can feed it, but rightclicking the horse afterwards will just force the player to face south without changing his position.

Reloading the game won't solve the problem, but it seems like switching to pause menue for a few minute works. After that, the buggy horses all behaved like normal untamed ones again.