MTG is supposed to work well. Best thing for it is massaging it in. This will stimulate the hair folicles and increase growth
Pig oil is good to thicken out manes and grow them out. My friends has had cobs who she bought with awful manes and now they're lovely.
Cowboy magic I really love it
Coconut oil! Put it on the roots and it helps a lot. You can use mtg too I just prefer the coconut oil. Also braid your name and take it out once a week, run your fingers through it and braids again. (Tail too)
I use Cowboy magic mane and tail detangler and shine. It's not meant as a growth product but I think that it works amazingly for making the mane and tail grow, shine, and look healthy.