A shiny coat only comes from a good diet. No amount of brushing and polishing sprays will make an unhealthy horse shiney.
A balanced diet and appropriate worming regime are the two most important factors. These will make all the difference in a horse's condition if they are lacking already.
Various suppliments can assist in 'finishing' a coat, but note that many of these suppliments will only work if the horse is deficient in something. Which goes back to a balanced diet - if it's balanced, you won't need a suppliment.
Then of course there's the care from the outside. The sun can bleach the coat, making it lighter, coarse and brittle. I keep my horses in light cotton sheets through summer on the high UV days to prevent burning and bleaching of the coat.
Avoiding washing regularly - shampoo's strip oils from the coat, making them fluffy. It takes a few days for the oils to recuperate. So you don't want to be washing more than once every few weeks absolute maximum.
Grooming is good, a regular going over with a curry comb can make a big difference, it lifts all of the dead skin and hair, and massages the skin and muscles, encouraging the production of natural oils.
I also use Hi-Shine hot oil treatments on my horses, after every full bath they get a hot oil treatment to replentish oils. After every ride, they get sprayed down with diluted Hi-Shine.
- By the way - the horses in the original post are not the same horse.