I give mine Smartpak Smartcombo supplements. It is supposed to provide "ingredients for joint, hindgut, hoof & coat health." I feel like any ridden horse would probably do well with some joint supplement, hindgut is crucial, and of course "no hoof no horse." The coat thing is probably just a bonus, but when Pony shed his winter coat this year (he's been on the supplements regularly since late summer) his spring coat was blindingly shiny. He's just a really sleek, healthy, good-looking pony to begin with, though. Moonshine did not get as shiny, but her sparse mane seems to be filling in.
Also a ration balancer. Where I live we have high selenium and low iron in our soil, so I had to find one that had less selenium and more iron.
I bought flax to grind myself and put in their feed for the healthy fat, and I did that for maybe two weeks, but then I just kept forgetting.