Cut nettles - high iron content
Cleavers/stickie willie - cleanses blood and aids circulation - ideal for those disposed to laminitis
Hawthorn - tips ideally but mine will chew delicately around thorns on larger pieces and leave the twigs stripped - another good for lami prone/at risk fatties
Rosebay willowherb - very common prolific weed, but only prior to flowering and don't confuse with foxgloves (digitalis) if you don't know one tall purple plant from another

Hand picked/scythed long grass but nothing cut with a mower and for me personally no strimmed stuff either - often hard to tell what's caught up in the latter and the former is a killer
Any brassica leaves
Rosehips and twigs
Willow (aspirin)
Mine eat dried thistles if I scythe field edges and leave them a day lying in the field they go - but then I have Highland ponies

Young dandelions leaves (diuretic but also good for digestion)