Remember that pressed pills have a lot of fillers in order to press them together into pill shape. This makes them harder to digest and it takes a lot longer for the body to break them down and absorb the active ingredient, in this case biotin. Horses digestive tracts are pretty short. Here is a quote from Practical Horseman:
"In horses, digesta usually passes through the stomach within two hours, though it can be as short as 15–20 minutes. The faster digesta moves, the less efficient digestion processes may be."
So I would say, while it probably would not harm your horse it would probably be a waste of money. Most of it would get pooped out I would think.
Riding: The art of keeping a horse between you and the ground. ~Author Unknown