The title says it all. I use MTG, and I'm careful not to brush hard pulling hairs out. Is this as good as it's going to get? LOL.
There's nothing to put on it that's going to get it to grow faster or give you more hair. People have been selling things to grow hair on eggs for centuries, but the eggs are still bald
(so save your money).
They have how ever many hair follicles they have and that's all their going to have.
Diet can have some impact, but not really to speed it up unless the diet is already slowing it down and then a better diet is just going to allow it to grow as much as it would normally.
There is only one thing that gives horses more (or less) hair and that's genetics so you really have no control over what the potential growth is going to ever be.
My current two come from much the same genetic pool (common grandfathers and great grandmothers from the mothers....same great grandfather father's side). They are first cousins maternally and second cousins paternally. One has massive (a real handful of hair) forelocks with some hair reaching her nostrils at times while the other has maybe half as much hair on her forelock and it's maybe 6-8 inches long. They both have massive manes and tails to the ground (the one with the short forelock has the slightly thicker mane and tail) They're closely related, same diet and same care, but you can't change the genetics they're born with.
Now all that being said, there are things you can do that will over time (a LONG time) can help some.
Diet and good health is the only way to reach whatever their max potential is. Coconut oil is great for the health of the hair. I won't make it grow faster or thicker (nothing will), but it helps for healthy hair and you don't have to wash it out (it gets absorbed). You're only real option (beyond diet and health) is to cut down on loss (hairs pulled out or broken) which is what all these "wonder" products are actually hoping to do (while claiming their helping "grow" hair) by making the hair slick.
Comb it out carefully (the oil helps here too by making it slick to help work out tangles and comb smoothly). Separate what you can by hand. For combing/brushing start near the ends and grasp the hair above it (to prevent pulling hairs out) and gently comb it out to cut down on breakage.
Sorry, but you're not going to see a lot of quick results. If the genes say the hair is going to grow 1" a year nothing is going to change that. It will be the best you can hope for so the best you can do is protect that 1".
You can spend money on MTG and other products and convince yourself that any progress you see is the result of using those products
(If you feel you MUST buy something to make you think the hair will grow more I suggest tea tree oil....at least it is good for the skin and kills fungus, etc...so while it won't grow the hair and probably isn't needed for all that skin, it does at least have a useful, although probably not needed, function