In what way would a wild flower meadow not be horse friendly?
Surely horses have been grazing this sort of natural meadow for centuries.
Yes, I do realise some plants are potentially more likely to cause harm than others, but "poisonous" in a natural mixture is more due to a plant's
potential to poison if taken in sufficient quantity, than its
likelihood to cause death at first nibble.
Think about it logically - salt is poisonous if taken in sufficient quantity, yet we all eat a certain amount of it in or on our food every day.
I was watching my donkeys yesterday eat a plant that was quite prevalent in the dry pasture they are on at present. I looked it up and was informed it contains oxalic acid and causes scouring if eaten (and presumably therefore would be classed as poisonous.) They have been nibbling it for some time, amongst the other vegetation they browse on, yet it has done them no harm because it is sufficiently diluted, as would any other individual variety be in a seed mix.
So, to me, it's a matter of degree. You aren't, presumably, planning on growing a monoculture of "poisonous" plants, but a mixture of which by far the dominant seeds are grass. By all means check that there's nothing you would consider to be totally toxic, but I personally wouldn't be too worried.