Have never heard of anything to feed a horse for scarring, however,
I have tried many products to help the healing process, including fresh bacon fat.
Have come across a product called "ABC JELL" from a health food shop. Not sure if ou can get it where you are from, it is wonderfull stuff, but takes time.
My old boy cut his back leg, on a fence, have applied the jell, hair is growing back, normal colour, scars has reduced by 50 per cent all ready, was unsure if I could show him again, by the looks of it I will be able too.
(he has won many champion ribbons in led and ridden)
This product got rid of a cancerous growth on a horse that the vets wanted to put down, the growth fell off. Horse is now doing dressage.
If you can get it, it is worth giving it a try, cost in Australia is about $20.00
Hope this helps.