Most 'weeds' in the UK can be fed to horses but should be treat like herbs as all can be dangerous if over fed or used as a grass replacement, the only one that I dont think causes problems in relatively large quanities are nettles.
Many of them are heavy detox on the liver and kidneys, things like hawthorn can be dangerous if the horse has anything wrong with its heart as it is a strong cardiac medicine but depending on what is wrong with the heart can cure or make worse.
Best thing to do is to offer small amounts of a few different ones and let them choose which one their bodies need this is what 'zoo pharma' folk get paid to come to your stables to do.
I wrote an article on nettles for horses a while back, they are great for OAP's, those with joint problems and also if you want to bring the dappling out in a coat
Fruit branches i know are okay for rabbits if not given to much, not sure how similar horses and rabbits are other than that neither can be sick
Its is really great to read that you keep horses and goats together, im thinking of doing the same in the future but have not met anyone yet who does this.