You cannot change genetics. Hair can only be as thick as the folicles allow.
MTG does NOT make hair grow. The bottle plainly says ENCOURAGES hair growth. Encourages does not equal make.
That's very true; but, it's worth a shot...there are things you can do to see if your horse's mane and tail have the potential to grow.
For me, Bandido must have the genetics for a beautiful mane and tail...but, I will say that MTG definitely sped up the growth of it.